Rashi on Genesis 18:4/רש"י על בראשית י״ח:ד׳

1יקח נא LET BE FETCHED, I PRAY THEE — This implies bringing by a messenger: therefore did the Holy One, blessed be He, recompense his (Abraham’s) children by a messenger when they required water, as it is said, (Numbers 20:11) “And Moses lifted up his hand and smote the rock etc.” (Bava Metzia 86b)

2ורחצו רגליכם AND WASH YOUR FEET — He thought they were Arabians who worship the dust of their feet, and he was particular not to have the object of idolatrous worship brought into his house (Bava Metzia 86b). Lot, however, who was not particular about this, mentioned “lodging” (i.e. entering the house) before “washing” the feet, as it is said (Genesis 19:2) “[And he said to the angels] tarry here all night and wash your feet.”

3תחת העץ UNDER THE TREE — beneath the terebinth (Genesis Rabbah 48:11).