TT #40 · Parashas Chayei Sarah · Passuk 24:60
And they blessed Rivkah and said to her, “O sister! May you grow into thousands of myriads; may your descendants seize the gates of their foes.”
Torah Temimah points to Kesuvos 7b. E-Daf · Sefaria — Kesuvos 7b
TT lemma: ויברכו את רבקה.
It was taught: Whence do we know that the blessing of bridegrooms is from the Torah? As it is said “And they blessed Rivkah.”
Custom (Bavli — Torah source for blessing the bride)
Blessing a bride (and by extension the wedding blessings) has a Torah basis in the blessing given to Rivkah by her family.