Before Moses even had a chance to open his mouth, God commanded him to keep it closed. Targum Pseudo-Jonathan preserves the command in all its strangeness: "Cease from thy prayer, and cry not for them before Me; for I will let My anger burn like strong fire against them, and consume them, and I will make thee a great people" (Exodus 32:10).
Why forbid Moses to pray?
The sages found a hidden kindness in this prohibition. The classical midrashic tradition (Berakhot 32a, c. 500 CE) taught that God's command "cease from thy prayer" was itself an invitation. Why tell Moses to stop praying unless prayer could change the outcome? If the decree were truly final, no prayer could touch it — so why forbid the effort? The very prohibition revealed that prayer had power. God was, in effect, telling Moses: the door is open; if you push, it will swing.
And then there is the offer at the end of the verse — "I will make thee a great people." God was offering Moses the deal Abraham would have leapt at: your descendants will replace the unfaithful nation. A new Israel, traced from Moses's own line. The previous Israel erased.
The sages admired Moses most for what he did next — which was to refuse both the prohibition and the offer. He did not cease from prayer. He did not accept the new nation. He chose, instead, to bind his fate so tightly to the doomed people that God could not destroy them without destroying him.
This is the template for Jewish intercessory prayer. You stand in the breach. You refuse the bargain that would save only you. You insist that the covenant must hold even for those who broke it — because if the covenant doesn't hold for them, it doesn't hold for anyone.
The Maggid takes this home: when God tells you to stop praying, keep praying. The command itself is the clue.