וַיָּ֤שֶׂם הָעֶ֙בֶד֙ אֶת־יָד֔וֹ תַּ֛חַת יֶ֥רֶךְ אַבְרָהָ֖ם אֲדֹנָ֑יו וַיִּשָּׁ֣בַֽע ל֔וֹ עַל־הַדָּבָ֖ר הַזֶּֽה׃
И слуга положил руку под бедро Авраама, господина своего, и поклялся ему по этому вопросу.
Chizkuni
תחת ירך אברהם, “under the thigh of Avraham.” Joseph, when asked by his father to swear that he would not bury him in Egypt but with his father and grandfather in Machpelah, did not do so, as he considered such behavior not respectful toward his father, even though he had requested him to swear. (Compare Genesis 47,2931)
Rashbam
על הדבר הזה, concerning the condition stipulated by Avraham.
Sforno
וישבע לו על הדבר הזה, he accepted all of Avraham’s conditions as part of his oath.