They wanted to tempt him to make pronouncements that would clash with the law. Jesus does not fall into their snare. He makes evident their hidden weapon by making them say themselves what Moses has commanded. The wording in Mark 10:3 is striking: Jesus speaks about Moses’ commandments as if they stand beside his own words (“What did Moses command you?”). Here there is already a preparation for what will follow in Mark 10:5: Moses’ command is typically the type of command that you (unfortunately) need.1
3 He answered them, “What did Moses command you?”