Musnad Ahmad 25:464

'Affan narrated to us, Abu al-Ahwas narrated to us, he said: Simak informed us, from 'Ikrimah, he said: Ibn 'Abbas said: I was approached while I was sleeping during Ramadan, and it was said to me: "Indeed, tonight is the Night of Decree (Laylat al-Qadr)." He said: So I stood up while I was drowsy, and I clung to some of the ropes of the tent of the Messenger of Allah, peace and blessings of Allah be upon him. I came to the Messenger of Allah, peace and blessings of Allah be upon him, and behold, he was praying. He said: So I looked into that night, and behold, it was the twenty-third night.