Joshua 21:7
"The children of Merari by their families had out of the tribe of Reuben, and out of the tribe of Gad, and out of the tribe of Zebulun, twelve cities."
Cross References (ranked by relevance)
Exodus 6:19
And the sons of Merari; Mahali and Mushi: these are the families of Levi according to their generations.
1 Chronicles 6:63
Unto the sons of Merari were given by lot, throughout their families, out of the tribe of Reuben, and out of the tribe of Gad, and out of the tribe of Zebulun, twelve cities.
Numbers 3:20
And the sons of Merari by their families; Mahli, and Mushi. These are the families of the Levites according to the house of their fathers.