“Fear not, Abram,
I am a shield to you;
Your reward shall be very great.”
What was Avraham afraid of?
Compare the source from Pirkei Avot and the source from Masseceht Pesacim
(ג) אַנְטִיגְנוֹס אִישׁ סוֹכוֹ קִבֵּל מִשִּׁמְעוֹן הַצַּדִּיק. הוּא הָיָה אוֹמֵר, אַל תִּהְיוּ כַעֲבָדִים הַמְשַׁמְּשִׁין אֶת הָרַב עַל מְנָת לְקַבֵּל פְּרָס, אֶלָּא הֱווּ כַעֲבָדִים הַמְשַׁמְּשִׁין אֶת הָרַב שֶׁלֹּא עַל מְנָת לְקַבֵּל פְּרָס, וִיהִי מוֹרָא שָׁמַיִם עֲלֵיכֶם:
(3) Antigonus a man of Socho received [the oral tradition] from Shimon the Righteous. He used to say: do not be like servants who serve the master in the expectation of receiving a reward, but be like servants who serve the master without the expectation of receiving a reward, and let the fear of Heaven be upon you.
Like servants who serve ... for the sake of receiving a payment God and religion should not be viewed as a vending machine in which you insert worship and obedience in order to get the good things in life. It is possible that Antigonus was impressed by the Hellenistic philosophical critique that everyday religionists are in it only for the payoff and that this constitutes a primitive religious position. He wanted Judaism to be a religion practiced at the highest level of religious motivation. The sages believed that the renewed self- limitation of God which spelled the end of visible miracles and prophecy came from God's desire that humans take a higher level of responsibility in the covenant process. To be worthy of this, individuals should shift their religious psychology from a focus on reward and punishment for doing or not doing God's bidding to a state of being motivated by one's relationship with God and a desire to do good for its own sake.
Answer
שיזכה לעולם הבא הרי זה צדיק גמור - והדתנן (אבות פ"א מ"ג) אל תהיו כעבדים המשמשין את הרב על מנת לקבל פרס היינו בכה"ג שאם לא תבוא לו אותה הטובה שהוא מצפה תוהא ומתחרט על הצדקה שעשה אבל מי שאינו תוהא ומתחרט ה"ז צדיק גמור וכן משמע בריש מסכת ר"ה (דף ד.) ובפ"ק דב"ב (דף י:).
Tosafot:
Doesn't it say in Avot (1:3) "Do not be as servants who are serving the master in order to receive a reward? This is referring to a situation when if the good that one hopes for does not come, one regrets the charity that was given. However, one who does not regret, such a person is completely righteous. This is also alluded to in tractate Rosh Hashana (4a) and the first chapter of Bava Batra (10b).
אל תהיו כעבדים המשמשין את הרב ע"מ לקבל פרס. פירש הר"ב אלא מאהבה בלבד. כלומר שעל זה הזהיר שתהא העבודה מאהבה בלבד לפי שזו היא עיקר העבודה אבל לא שהוא ר"ל שהעבודה שעל מנת לקבל פרס היא אסורה. שמ"מ עובד ה' הוא. ומפני כן לא נמנעה התורה השלמה להודיע השכר והעונש. אלא שאם יעבוד לתקות שכר או מניעת ההיזק. הנה אינו במעלה כמו העובד מאהבה שאין לו בעבודתו שום פנייה כלל. ורצה התנא להזהירנו שנהיה מן העובדים מאהבה. אבל לא שיוציא העובד לתקות שכר מכלל העובדים. לא כן הוא. וכמאמרם בפ"ק דב"ב [דף י' ע"ב] האומר סלע זו לצדקה בשביל שיחיה בני. כדי שיהיה לי חלק לעולם הבא. הרי זה צדיק גמור. ועוד שאם אי אתה אומר כן אתה מקפח ח"ו רגליהן של שונאי ישראל. כי איה שוקל איה סופר אשר יאמר כי הוא זה העובד מאהבה. הייטב בעיני ה' שכל העובד ומתכוין שיתקיים בו יעודו התורה שנאמרה לנו מפיו ב"ה שלא יהיה מקבל שכרו הטוב על זה. לא ניחא למרייהו דתימרו הכי...
וראיתי לרבינו מהר"ר ליווא מפראג ז"ל בספר דרך חיים שכתב שלכך אמר אלא הוו כעבדים וכו' ולא די לו באמרו אל תהיו וכו' לפי שממאמרו אל תהיו היה נשמע ששולל ואוסר מלהיות כעבדים המשמשים וכו' וזה אינו שאין זה איסור שגם העובד כן הוא צדיק גמור. אבל לא בא אלא להורות שיש עוד מעלה ומדרגה יותר גדולה והיא העובד מאהבה. ולכך הוצרך לכפול ולומר אלא הוו כעבדים וכו' לומר שאין תכלית כוונתו אלא שתהיו כעבדים המשמשין שלא ע"מ וכו'. וזו היא המעלה היותר גדולה. אבל אין רצונו לומר לאסור לשמש ע"מ לקבל פרס. אלו דבריו ודעתו ז"ל...
ומעתה על אחת כמה וכמה שלא ישמע ע"פ דבריו של התנא טעות צדוק וביתוס שהיו אומרים שנתכוין לומר שאין כאן קבול פרס כלל כמו שנזכר בדברי הר"ב בפי' משנה י"א. אלא שהוה ליה לפרש בפירוש שיהא מפורש לכל שומע.
BE NOT LIKE SERVANTS SERVING THE MASTER FOR PAY. Rav: rather, out of love alone. In other words, the mishna enjoins that the service be out of love alone, for this is the truest kind of service. It does not mean to say, however, that that service for reward is forbidden, as in either case he is serving God; for this reason our perfect Torah did not see unfit to mention rewards and punishments. One who serves in the hopes of earning reward or avoiding pain is simply not on the level of the one who serves out of love alone, whose service has no secondary intention whatsoever.
And the tanna wished to enjoin us to be among those who serve out of love alone, but not to exclude one who serves in the hopes of earning reward from the category of those who serve! This is not the case, as the Sages say in Bava Batra 10b: One who says, “I give this sela to charity that my son should live!” or “...that I should have a share in the world to come!” is perfectly righteous. Furthermore, if one takes the opposite position, one cuts the very legs out from under the nation of Israel! For “where is the weigher, where is the counter” (Isaiah 33:18) who shall say that so-and-so is one who truly serves out of love alone? And can it be good in God’s eyes that anyone who serves Him with the intent that the rewards promised by the Torah itself be fulfilled—that same Torah which has been communicated to us from His mouth—that such a one not receive his good lot on account of this? It pleases not their Master that people should say thus!
I saw that our master Maharal, in his commentary on Avot, Derech Chaim, deals with the question of why the mishna continues and says “but rather be like servants who serve the master without thought of pay” and doesn’t simply stop at “be not like servants serving the master for pay". He writes that had the Tanna said only “be not like servants serving the master for pay” the implication would have been that he prohibits being like servants serving the master for pay, while in fact there is no such prohibition and one who does so is even completely righteous. He means only that there is a level and grade above that one, which is serving out of love alone. It is for this reason that he goes back and explicitly says “rather, be like servants etc.”, to explain that he means only that one should aspire to be a servant serving without thought of pay. All this is the opinion of Maharal.
At this point, there is certainly no way to misunderstand the words of the tanna the way Tzadok and Baitos did, who claimed that he meant to say that there is no pay to be received at all, as Rav mentions in his commentary to mishna 11. Still, he should have made it completely clear to the point that whosever was listening would have understood this.
Another Answer:
Avraham was not at all concerned with reward for his own sake - he was worried about using up Zechut Avot.
אָנָּא מֶֽלֶךְ חַנּוּן וְרַחוּם זְכוֹר וְהַבֵּט לִבְ֒רִית בֵּין הַבְּ֒תָרִים וְתֵרָאֶה לְפָנֶֽיךָ עֲקֵדַת יָחִיד לְמַֽעַן יִשְׂרָאֵל:
Mon. Thurs. Shacharit
We beseech You, gracious and compassionate King, remember and look to the Covenant between the divided pieces, and let the binding [upon the Altar] of the only son appear before
זְכֹר בְּרִית אַבְרָהָם וַעֲקֵדַת יִצְחָק. וְהָשֵׁב שְׁבוּת אָהֳלֵי יַעֲקֹב וְהוֹשִׁיעֵנוּ לְמַעַן שְׁמֶךָ:
Avot de-Rabbi Nathan (Hebrew: אבות דרבי נתן)(c.700–900 CE).
Antigonus, a man of Sokho, received from Shimon the Righteous. He would say: Do not be like servants who attend upon their master only on the condition that they receive a reward. Rather, be like servants who attend upon their master only on the condition that they do not receive a reward. And let the awe of the heavens be upon you so that your compensation will be double in the future.

(א) רַבִּי אוֹמֵר, אֵיזוֹהִי דֶרֶךְ יְשָׁרָה שֶׁיָּבֹר לוֹ הָאָדָם, כֹּל שֶׁהִיא תִפְאֶרֶת לְעוֹשֶׂיהָ וְתִפְאֶרֶת לוֹ מִן הָאָדָם. וֶהֱוֵי זָהִיר בְּמִצְוָה קַלָּה כְבַחֲמוּרָה, שֶׁאֵין אַתָּה יוֹדֵעַ מַתַּן שְׂכָרָן שֶׁל מִצְוֹת. וֶהֱוֵי מְחַשֵּׁב הֶפְסֵד מִצְוָה כְּנֶגֶד שְׂכָרָהּ, וּשְׂכַר עֲבֵרָה כְנֶגֶד הֶפְסֵדָהּ. וְהִסְתַּכֵּל בִּשְׁלשָׁה דְבָרִים וְאִי אַתָּה בָא לִידֵי עֲבֵרָה, דַּע מַה לְּמַעְלָה מִמְּךָ, עַיִן רוֹאָה וְאֹזֶן שׁוֹמַעַת, וְכָל מַעֲשֶׂיךָ בַסֵּפֶר נִכְתָּבִין:
(1) Rabbi Said: which is the straight path that a man should choose for himself? One which is an honor to the person adopting it, and [on account of which] honor [accrues] to him from others. And be careful with a light commandment as with a grave one, for you did know not the reward for the fulfillment of the commandments. Also, reckon the loss [that may be sustained through the fulfillment] of a commandment against the reward [accruing] thereby, and the gain [that may be obtained through the committing] of a transgression against the loss [entailed] thereby. Apply your mind to three things and you will not come into the clutches of sin: Know what there is above you: an eye that sees, an ear that hears, and all your deeds are written in a book.
And Rabbi certainly did not disagree with Antigonos of Socho’s teaching that one shouldn’t be like a servant serving G-d for pay. He meant, rather, that when you are accosted by the “vile one”—he who is called satan and the “evil inclination”—consider loss and reward in order to deflect him—Abarbanel and Maharal in Derech Chaim.
Now according to my commentary on the mishna of Antigonos (Avot 1:3), where I write that Antigonos did not mean that those who serve in order to receive reward are not really servants of G-d, we can say that Rabbi’s approach is for the greater part of the people, and Antigonos’ is for the very few. If only the greater part were just as the few!
