who.

John 1:15, 30
John testified concerning Him. He cried out, saying, "This is He of whom I said, 'He who comes after me has surpassed me because He was before me.'" . . .

Acts 19:4
Paul explained: "John's baptism was a baptism of repentance. He told the people to believe in the One coming after him, that is, in Jesus."

whose.

Matthew 3:11
I baptize you with water for repentance, but after me will come One more powerful than I, whose sandals I am not worthy to carry. He will baptize you with the Holy Spirit and with fire.

Mark 1:7
And he began to proclaim: "After me will come One more powerful than I, the straps of whose sandals I am not worthy to stoop down and untie.

Luke 3:16
John answered all of them: "I baptize you with water, but One more powerful than I will come, the straps of whose sandals I am not worthy to untie. He will baptize you with the Holy Spirit and with fire.