Or HaChaim on Genesis 18:4/אור החיים על בראשית י״ח:ד׳

1 יקח נא מעט מים. "Let a little water be brought." Perhaps the reason Abraham spoke about a "little" water was to indicate that it was not much of an effort to procure it. He certainly was not miserly. Apparently, he did not want to depart from his custom to ask guests to wash their feet to remove the dust that most of them worshipped (Baba Metzia 86). [The author felt the need to justify the custom seeing Abraham was already aware that his guests were angels and not given to worship the dust. Abraham may not have wanted his servants to realise that his guests were angels. Ed.]

2Abraham may have hinted to the angels that the water he spoke about was a reference to the Torah. Torah is multi-faceted; it speaks to us as פשט, plain meaning of the words, and it also contains messages on a far deeper level. Abraham faced heavenly beings in human guise, i.e. G'd had made a body for them through solidifying something normally ethereal. When an angel assumes human form his "body" is compared to the "foot". This is a concept familiar to students of the Kabbalah (Zohar 1,58). Abraham hinted to the angels that they should take a little of the plain meaning of the Torah i.e. "water," seeing that they were presently in human form. They could then rest under the "the Tree," i.e. hyperbole for Torah which is known as עץ החיים. He referred to it as העץ, though it had not previously featured in our story. He also told them to take פת לחם, a reference to the inner meanings of Torah, something he alluded to when speaking about their לבכם, inner organs. He invited the angels to enjoy the inner aspects of Torah. [This is not so strange-sounding, seeing angels had never been given the Torah. Ed.] The reason he did not merely say פת but פת לחם, was a hint that this bread had inner properties, i.e. its numerical value being 78, or three times the numerical value (26) of the four-lettered Holy Name. This was an allusion to eternal life, i.e. past, present and future.