The twelfth book of Numbers talks about how Aaron and his wife, Miriam, spoke negatively about Moses for marrying a Cushite. When God heard their gossip, He was angered and spoke to all three of them, telling Aaron and Miriam that they should not dare speak against Moses, who is His chosen servant. Numbers 12:9-13 reads:
“The anger of the Lord burned against them, and he left them. When the cloud lifted from above the tent, Miriam’s skin was leprous[a]—it became as white as snow. Aaron turned toward her and saw that she had a defiling skin disease, and he said to Moses, “Please, my lord, I ask you not to hold against us the sin we have so foolishly committed. Do not let her be like a stillborn infant coming from its mother’s womb with its flesh half eaten away.” So Moses cried out to the Lord, “Please, God, heal her!””
The passage above talks about how Aaron and Miriam were punished by God for gossiping against Moses. Seeing Miriam’s skin disease, Aaron begged Moses to forgive them. At this, Moses immediately pleaded that God heal her.