Mark 1:7
And preached, saying, There cometh one mightier than I after me, the latchet of whose shoes I am not worthy to stoop down and unloose.
Cross-reference
Matthew 3:11 parallels John's proclamation of a mightier one and adds baptism with the Holy Spirit and fire.
Luke 3:16 has the same saying about unworthiness to untie sandals, reinforcing John's humility before Christ.
John 1:27 directly quotes John's statement about unworthiness to untie the sandal, a clear parallel.
Acts 13:25 quotes John's same humble confession about untying sandals, confirming his role as forerunner.
John 1:15 records John's testimony that the one coming after him ranks before him, reinforcing his humility.
Matthew 3:14 shows John's humility in practice when he protests baptizing Jesus, consistent with his sandal statement.
John 3:28-31 expands John's testimony that he is not the Christ but sent before Him, deepening the context of his humility.
John 1:20 has John explicitly deny being the Christ — consistent with his humility in Mark 1:7.