Quotation for Kiddushin 64:17

"and Aaron and all the elders of Israel came to eat bread with Moses' father-in-law before G d": And where did Moses go? Did he not go out to him in the beginning? viz. (Ibid. 3) "and Moses went out to his father-in-law." We are hereby apprised that he stood and waited upon them. Whence did he learn (to do this)? From our father Abraham. They said: This thing was expounded by R. Tzaddok, viz.: When R. Gamliel made a feast for the sages, all the sages of Israel were seated before him and R. Gamliel arose and served them, whereupon they said: We are not worthy of this. At this, R. Yehoshua said to them: Let him serve us. We find one greater than R. Gamliel who served men. They: Who? R. Yehoshua: Our father Abraham, the greatest in the world, who served ministering angels thinking them to be Arab idolators. How much more so (should this be done by) R. Gamliel, serving sages, students of Torah! At this, R. Yitzchak said: We find one greater than R. Gamliel and than Abraham, who served men. They: Who? R. Gamliel: The Shechinah, who at all times supplies food for all who enter the world in accordance with their needs, and sates all living things with good will. And not only men who are worthy and tzaddikim, but even evildoers, idolators! How much more so should R. Gamliel pay attendance upon sages and Torah scholars!

And once it happened that R. Eliezer and R. Yehoshua and R. Tzaddok were seated at the (wedding) feast of R. Gamliel's son, when R. Gamliel poured a cup (of wine) for R. Elazar, who declined taking it, and for R. Yehoshua, who took it — whereupon R. Elazar said to him: What is this, Yohoshua? We are seated and R. Gamliel stands over us and serves us? R. Yehoshua: Let him serve. Abraham, the greatest man in the world served the ministering angels, (who came to visit him), thinking them to be Arab idolators, and R. Gamliel should not serve us? R. Tzaddok: You have forsaken the honor of the L-rd, and you occupy yourselves with the honor of flesh and blood! He who spoke and brought the world into being causes the winds to blow, and raises vapors and clouds and brings down rains and nurtures growths and sets a table for each and every man — and Gamliel should not serve us?