Luke 20:5

And they reasoned with themselves, saying, If we shall say, From heaven; he will say, Why then believed ye him not?

Cross-reference

In John 5:33-35, Jesus affirms John’s testimony, which the leaders in Luke 20:5 are debating. Their rejection of John is a refusal of that light.

Matthew 21:25 is the synoptic parallel, giving the identical debate among the leaders about John’s baptism being from heaven or men.

Acts 13:25 records John’s own confession of his subordinate role to Christ, the very point the Pharisees are evading in Luke 20:5.

John 1:30 reiterates John's confession of Jesus' superiority, confirming John's prophetic role and why the people accepted his baptism.