In the account of Jesus' temptation in Luke 4, I've been pondering why in Jesus' third response to the devil, rather than saying -- as in the first 2 times -- "it is written," he says, "it is said." ("And Jesus answering said unto him, It is said, Thou shalt not tempt the Lord thy God.")
Here's Lenski's take on the question. I found it helpful:
"After the devil himself used, "it has been written," in imitation of Jesus who used this formula, Jesus now says pointedly, "it has been said," namely by God himself. His word was, of course, also written, but Jesus stresses the fact that God himself spoke this word, and he is certainly able to speak his meaning so as to make it clear ... Jesus does not set one Scripture passage against another. Jesus places one Scripture passage beside another, each casts light on the other. He thus establishes the great principle of all correct interpretation: Scriptura ex Scriptura explicanda est, Scripture is explained and must be explained by Scripture. We dare not put our own or any other man's ideas into it. Any false conclusions or deductions drawn from any one passage are eliminated by comparing this with other pertinent passages. No man dare force into a passage a thought that contradicts another passage. This condemns all exegesis that operates with contradictions in the Scriptures. In the present case all is clear: Ps. 91:11, 12 dare not be stressed so as to clash with Deut. 6:16, 'Thou shalt not test out the Lord, thy God.'"