Adapted from the teachings of the Rebbe by Yanki Tauber
There is a story about the famous Reb Levi Yitzchak of Berdichev who, as a young unknown scholar, found himself stranded in a small town just before Shabbat began. He went from house to house seeking lodgings but he was turned away each time.
Finally, at a dilapidated old house at the very edge of the town, Reb Levi Yitzchak again asked for lodging. The man who answered the door enthusiastically agreed that Reb Levi Yitzchak could stay, but warned him that as a poor man he had very little to offer. Reb Levi Yitzchak accepted the modest offer, and after Shabbat bid the man farewell.
Years later when Reb Levi Yitzchak was known far and wide as the great sage of Berdichev, he found himself again in the very same town. This time, however, he was recognized and families were fighting over who would host the great sage.
Reb Levi Yitzchak settled the matter very quickly, "I will stay at the very same home, at the edge of town, with the poor man that I stayed with years ago, if he will accept me. It was he who took me in the last time I stayed here and I must express my everlasting gratitude.
Once someone performs a favor for you, you must never forget it.