2 Samuel 5:11

Then came all the tribes of Israel to David unto Hebron, and spake, saying, Behold, we are thy bone and thy flesh.

Also in time past, when Saul was king over us, thou wast he that leddest out and broughtest in Israel: and the LORD said to thee, Thou shalt feed my people Israel, and thou shalt be a captain over Israel.

So all the elders of Israel came to the king to Hebron; and king David made a league with them in Hebron before the LORD: and they anointed David king over Israel.

David was thirty years old when he began to reign, and he reigned forty years.