Where does it say in the Bible that the apostle Paul was baptized?

A disciple of Damascus named Ananias.

Acts 9:17, 18 - And Ananias went his way and entered the house; and laying his hands on him he said, "Brother Saul, the Lord Jesus, who appeared to you on the road as you came, has sent me that you may receive your sight and be filled with the Holy Spirit." Immediately there fell from his eyes something like scales, and he received his sight at once; and he arose and was baptized. [NKJV]