Musar do Rodzaju 18:3

ויאמר אדני אם נא מצאתי חן בעיניך אל נא תעבור מעל עבדך . We learn from here that when one tries to do someone a favor one should describe it in such a way that the recipient feels it is he who is doing the favor by accepting same. When one creates such an atmosphere, the recipient will not decline; he will not think that the donor will boast about having done him a particular favour. This is why Abraham described himself as in those people's debt if they honoured him with their presence.

This is what the Talmud Shabbat 127 has in mind when it states that the entertaining of guests is an even greater accomplishment than the welcoming of G–d's Presence since the Torah reports that Abraham asked G–d to wait a few moments: אל נא תעבור אד-ני, אם נא מצאתי חן בעינך מעל עבדך, while he would attend to the guests who had arrived suddenly (18,3).

For instance, he said: "my lords, if I have found favor in your eyes do not pass by your servant." Our sages already are divided in their opinions as to the meaning of the word א-ד-ניי here. Some say that Abraham addressed the שכינה, whereas others say that he addressed the leader of the three strangers. If the former, the word is sacred, if the latter, it is ordinary. Both opinions present difficulties. According to the view that the word is used as an appellation for G–d's Presence, the word should have been written before "he ran towards them." If, however, the words were addressed to the leader of the three men, and the word is ordinary, the vowel under the letter נ should have been a Chirik, and not a Kametz. There is no point in assuming that our sages disagreed as to the vowel that should appear under the letter נ.

Now we understand that even if the appellation was meant to be ordinary, addressed to humans, the vowel kametz was still in order, since, though Abraham addressed only the leader of the three men directly, the others were included in Abraham's invitation. Proof of this is the plural ending that Abraham used when saying ורחצו רגליכם, "and wash (pl) your feet" (18,4). To the leader Abraham said: "Do not go away, neither you nor your companions; you are all considered by me as my lords. But you, the leader are my special lord, seeing that you are also your companions' lord." In this manner the use of the vowel kametz is justified even though the word is not used in the sacred sense.