Musar do Rodzaju 18:8

ויאכלו . Bereshit Rabbah 48,14 quotes Rabbi Tanchum saying in the name of Rabbi Meir that we can learn manners from here. When Moses went to heaven (Mount Sinai and beyond), he mentioned that he had not eaten or drunk for forty days and nights. This was because heavenly beings do not require food and drink, and Moses conducted himself according to local custom. On finding themselves in an earthly environment, the angels behaved accordingly and accepted food and drink. [Bamidbar Rabbah 10,5 explains that the reason why the angel who appeared to Manoach refused to eat whereas the angels that visited Abraham accepted food and drink is simple. The latter had appeared in the guise of humans, whereas the former had never appeared in the guise of a human being. Ed]

The verse continues saying that "Abraham stood over them under the tree while they ate." We must understand this as being similar to Exodus 24,11: ויחזו את האלוקים ויאכלו וישתו, "They had a vision of G–d and ate and drank." Abraham stood by them in order to introduce them to monotheism and an appreciation of the kindness and love of G–d for His creatures. While all this transpired, Abraham still had not resolved his doubts as to the true identity of his guests.