8:17-18 What scripture is Jesus quoting here?

Jesus is quoting De 17:6 and 19:15 here (CP De 17:6; 19:15).

Two witnesses establishes the truth of any testimony (CP 2Cor 13:1; 1Ti 5:19; He 10:28).

Jesus was not His own witness as the Jews contended (CP Jn 8:12-16).

Not only God, but the Holy Spirit also testified to Jesus' Deity (CP 1Jn 5:7).