Genesis 21:22-24

"And it came to pass at that time, that Abimelech and Phicol the captain of his host spake unto Abraham, saying, God is with thee in all that thou doest: now therefore swear unto me here by God that thou wilt not deal falsely with me, nor with my son, nor with my son's son: but according to the kindness that I have done unto thee, and to the land wherein thou hast sojourned. And Abraham said, I will swear."

Abraham's increasing power and prosperity had evidently raised a certain fear in the mind of Abimelech that future conflict might develop between them. Therefore Abimelech sought by honorable and peaceful means to guard against any such possibilities. Abraham promptly took the requested oath, remembering, no doubt, that Abimelech had indeed granted manifold favors to him, including the rich gifts upon the occasion of his intended marriage to Sarah. Abraham seized upon the occasion to resolve a conflict over possession of a well which had been claimed by some of Abimelech's servants.