1 Kings 2:11

The days that David reigned over Israel were forty years; he reigned seven years in Hebron, and he reigned thirty-three years in Jerusalem.

2 Samuel 5:4-5

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

1 Chronicles 29:26-27

Thus David, the son of Jesse, reigned over all Israel.

Treasury of Scripture Knowledge

Summary

And the days that David reigned over Israel were forty years: seven years reigned he in Hebron, and thirty and three years reigned he in Jerusalem.

Bible References

Reigned over

2 Samuel 5:4
David was thirty years old when he began to reign, and he reigned forty years.
1 Chronicles 29:26
Thus David, the son of Jesse, reigned over all Israel.