תּלמוד אויף בראשית 24:60

A Kallah is forbidden to her husband without Birchas Chasanim in the same manner as a Niddah. Just as a Niddah who has not immersed [In Mikva] is forbidden to her husband, so likewise is a Kallah forbidden to her husband without the [marriage] brachah. From where do we know Birchas Chasanim is derived from the Torah? As it said, “And they blessed Rivkah...” (Bereshis 24:60). From where do we derive that even a widow is forbidden? As it is said, “He then took ten elders of the city…Then all the people who were at the gate, said, [we are] witnesses!...May HASHEM make the woman...May your house be like the house of Peretz...” (Rus 4:11-12, 2).