Who said of Jesus, "He must increase; I must decrease"?

Answer: John the Baptist

When told everyone was going to Jesus, John the Baptist said he was the friend of the bridegroom: "He must increase; I must decrease."

“So John’s disciples came to him and said, “Look, Rabbi, the One who was with you beyond the Jordan, the One you testified about—He is baptizing, and everyone is going to Him.””— John 3:26 (BSB)