Reference for Genesis 24:9

וַיָּ֤שֶׂם הָעֶ֙בֶד֙ אֶת־יָד֔וֹ תַּ֛חַת יֶ֥רֶךְ אַבְרָהָ֖ם אֲדֹנָ֑יו וַיִּשָּׁ֣בַֽע ל֔וֹ עַל־הַדָּבָ֖ר הַזֶּֽה׃

And the servant put his hand under the thigh of Abraham his master, and swore to him concerning this matter.