Musnad Ahmad 25:688

'Affan narrated to us, Abu al-Ahwas narrated to us, and he said: Simak informed us, from 'Ikrimah, who 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, may Allah bless him and grant him peace, and behold, he was praying." He said: "So I looked into that night," and he said: "And behold, it was the twenty-third night."