What Old Testament event is Romans 9:15 referring to?

Romans 9:15 refers to the Old Testament episode of the golden calf, when Moses was flat on his face before God. At that time God declared to Moses, 'I will have compassion on whom I will have compassion, and I will have mercy on whom I will have mercy,' asserting His sovereign authority.

Les explains that Romans 9:15 points back to when Moses was flat on his face after the episode of the golden calf. There God told Moses, 'I will have compassion on whom I will have compassion, and I will have mercy on whom I will have mercy,' because He is sovereign, the final authority. Paul then writes in verse 16 that it is not of him that willeth, nor of him that runneth, but of God who showeth mercy.

Romans 9:15

For he saith to Moses, I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion.

What event in Moses' life is Romans 9 referring to when it speaks of God's compassion and mercy?

Romans 9 refers to the golden calf incident, when Moses was flat on his face before God. In that setting God declared, "I will have compassion on whom I will have compassion, and I will have mercy on whom I will have mercy," asserting His sovereign authority to show mercy according to His own will.