Rashi on Genesis 18:5/רש"י על בראשית י״ח:ה׳

1וסעדו לבכם AND SUSTAIN YOUR HEARTS — In the Torah, the Prophets and the Hagiographa we find that bread is the sustenance of the heart. In the Torah. “[And I will fetch a morsel of bread] and sustain your heart.” In the Prophets: “Stay thy heart with a morsel of bread” (Judges 19:5). In the Hagiographa: “And bread that sustaineth man’s heart” (Psalms 104:15). R. Hama said: Here is not written לבבכם but לבכם thus teaching that the evil inclination has no power over angels (Genesis Rabbah 48:11)

2אחר תעבורו means after that, you may go away.

3כי על כן עברתם FORASMUCH AS YE HAVE PASSED —For (כי) I ask you this thing because that (על כן) you have honoured me by calling at my place. כי] על כן] has the same meaning as על אשר “because that”. So, too, is the meaning wherever כי על כן occurs in Scripture, e. g., (Genesis 19:9) “For (כי) I ask you this because that (על כן) they have come under the shadow of my roof”; (Genesis 33:10)

4“For (כי) you ought to do this because that (על כן) I have seen thy face”; (Genesis 38:26) “for (כי) she has done right because that (על כן) I gave her not”; (Numbers 10:31) “for (כי) thou ought to accompany us because that (על כן) thou knowest how we are to encamp”.