Midrasch zu Hoschea 12:7

Rabbi Simeon said: Abraham spake to Isaac his son (saying), This servant is suspected of all the transgressions of the Torah, and deceit is in this servant, as it is said, "He is a Canaanite, the balances of deceit are in his hand; he loveth to defraud" (Hos. 12:7). See, lest he has defiled her, therefore bring the girl into the tent and examine her tactually; || and if she be undefiled, behold, she is destined for thee from her mother's womb. He brought her into the tent and examined her tactually, and he showed the result to Abraham his father, and afterwards he took her to be his wife, as it is said, "And Isaac brought her into the tent of Sarah his mother… And Isaac was comforted after his mother's death" (Gen. 24:67); for the deeds of Rebecca were like unto those of Sarah. Hence the Israelites have the custom of producing the tokens of the damsel's virginity, as it is said, "Then shall the father of the damsel, and her mother, take and bring forth the tokens of the damsel's virginity" (Deut. 22:15).