Moses had married a Cushite woman, and Miriam and Aaron criticized him for it.
References for Numbers 12:1
They said, "Has the Lord spoken only through Moses? Hasn't he also spoken through us?" The Lord heard what they said
References for Numbers 12:2
(Moses was a humble man, more humble than anyone else on earth.) 1
References for Numbers 12:3
+212.3 Si 45.4.
Suddenly the Lord said to Moses, Aaron, and Miriam, "I want the three of you to come out to the Tent of my presence." They went,
and the Lord came down in a pillar of cloud, stood at the entrance of the Tent, and called out, "Aaron! Miriam!" The two of them stepped forward,
and the Lord said, "Now hear what I have to say! When there are prophets among you, I reveal myself to them in visions and speak to them in dreams.
References for Numbers 12:6