2 Samuel 2:10 (BSB)

Saul’s son Ish-bosheth was forty years old when he began to reign over Israel, and he reigned for two years. The house of Judah, however, followed David.

From 2 Samuel 2. Also in the ESV.

Commentary on 2 Samuel 2:10

• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on 2 Samuel 2:8-17: Here is, I. A rivalship between two kings - David, whom God made king, and Ishbosheth, whom Abner made king. One would have thought, when Saul was slain, and all his sons that had sense and spirit enough to take the field with him, David would come to the throne without any opposition, since all Israel knew, not only how he had signalized himself, but...
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on 2 Samuel 2:10: Ishbosheth, Saul's son, was forty years old when he began to reign over Israel,.... Being born the same year his father began to reign. See Gill on Sa1 31:6, and reigned two years; which some understand of these, and no more; and whereas David reigned seven years and a half over Judah, before he reigned over all Israel, it is thought by the Jewish chronologer...
• Adam Clarke (Methodist), Clarke's Commentary on the Bible on 2 Samuel 2:10: Ish-bosheth - reigned two years - It is well observed that Ish-bosheth reigned all the time that David reigned in Hebron, which was seven years and six months. Perhaps the meaning of the writer is this: Ish-bosheth reigned two years before any but the tribe of Judah had attached themselves to the interest of David.
• Keil & Delitzsch (Lutheran), Biblical Commentary on the Old Testament on 2 Samuel 2:10-11: 2Sa 2:10-11 Length of the reigns of Ishbosheth over Israel, and David at Hebron. The age of Ishbosheth is given, as is generally the case at the commencement of a reign. He was forty years old when he began to reign, and reigned two years; whereas David was king at Hebron over the house of Judah seven years and a half.