Matthew 26:16
And from that time he sought opportunity to betray him.
Cross-reference
Matthew 17:22
Prophetic fulfillment
Matthew 17:22 prophesies the Son of Man being delivered into men's hands — here Judas begins to fulfill that prediction.
Mark 14:11
Parallel
Mark 14:11 ends with Judas seeking opportunity to betray — directly parallel to Matthew 26:16's phrase 'sought opportunity'.
Luke 22:6
Parallel
Luke 22:5 describes the agreement to give money — the immediate precursor to Judas seeking opportunity in 26:16. Closely connected narrative.