John 1:6-8 describes John's divine mission as a witness to the light, directly matching the witness Jesus cites in this verse.

John 1:15-18 contains John's explicit witness that Jesus is pre-existent and the one who makes God known.

John 1:19-27 records John's specific testimony when questioned—denying he is the Christ and preparing the way for Jesus.

John 1:29-34 records John's witness identifying Jesus as the Lamb of God and Son of God—the core of his testimony.

John 3:26 reports that John's disciples acknowledged his prior witness to Jesus, confirming the testimony Jesus refers to here.

John 1:7 introduces John the Baptist as a witness to the light — exactly the testimony Jesus refers to here.

Matthew 21:32 shows the rejection of John's testimony — the same witness Jesus mentions here was not believed.

Acts 19:4 explicitly states John told people to believe in Jesus — the very testimony Jesus refers to here.