Albert Barnes Commentary 1 Kings 11:13

"Howbeit I will not rend away all the kingdom; but I will give one tribe to thy son, for David my servant`s sake, and for Jerusalem`s sake which I have chosen." — 1 Kings 11:13 (ASV)

One tribe—that is, the tribe of Judah. Benjamin was looked upon as absorbed into Judah, so it was not really a tribe in the same sense as the others. Still, in memory of the fact that the existing tribe of Judah was a double one (1 Kings 12:21), the prophet Ahijah tore his garment into twelve parts and kept back two from Jeroboam (1 Kings 11:30–31).