Hadith 10:892
Abu Bakr narrated to us, he said: Shababa narrated to us, he said: Shu'bah narrated to us, he said: I asked al-Hakam about the one who is appointed [as a ruler/guardian] over the Ummah. He said: Ibrahim said, "Its period is two months." And I asked Hammad, and he said the same as that.
حدثنا أبو بكر قال نا شبابة قال نا شعبة قال سألت الحكم عمن يولي من الأمة فقال قال إبراهيم عدتها شهران وسألت حمادا فقال مثل ذلك