Charles Ellicott Commentary Numbers 16:12

"And Moses sent to call Dathan and Abiram, the sons of Eliab; and they said, We will not come up:" — Numbers 16:12 (ASV)

And Moses sent to call Dathan and Abiram ... — If, as seems probable from Numbers 16:3, Dathan and Abiram had joined Korah and his company in the address to Moses which is contained in Numbers 16:3, they must have later withdrawn themselves and returned to their own tents, from which they refused to go up at Moses's command.