Moses prayed: "O my Lord! forgive me and my brother! admit us to Thy mercy! for Thou art the Most Merciful of those who show mercy!" Al-A'raf 7:151 ⧉
Tafsir:
When Moses came down from the Mountain and saw the Israelites worshipping the golden calf, he held Aaron responsible for this, under the impression that he had neglected his duty as a reformer. Moses caught hold of him in anger but Aaron clarified his position and told him that he had done all he could to show them the right path. But they had not listened to him. So Moses checked himself, and started praying to God for Aaron. Many misunderstandings take place between believers, but after clarification they return to normalcy, just as if no misunderstanding had ever taken place.