Keil & Delitzsch

When sitting, about mid-day, in the grove of Mamre, in front of his tent, Abraham looked up and unexpectedly saw three men standing at some distance from him (עליו above him, looking down upon him as he sat), viz., Jehovah (Gen 18:13) and two angels (Gen 19:1); all three in human form. Perceiving at once that one of them was the Lord (אדני, i.e., God), he prostrated himself reverentially before them, and entreated them not to pass him by, but to suffer him to entertain them as his guests: "Let a little water be fetched, and wash your feet, and recline yourselves (השּׁען( sevle to recline, leaning upon the arm) under the tree." - "Comfort your hearts:" lit., "strengthen the heart," i.e., refresh yourselves by eating and drinking (Jdg 19:5; Kg1 21:7). "For therefore (sc., to give me an opportunity to entertain you hospitably) have ye come over to your servant:" כּן על כּי does not stand for כּי כּן על (Ges. thes. p. 682), but means "because for this purpose" (vid., Ewald, 353).