Musnad Ahmad 25:1600

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from Ibn Tawus, from 'Ikrimah ibn Khalid, from Ibn 'Abbas, who said: I was in the house of Maymunah, and the Prophet, peace and blessings of Allah be upon him, stood up to pray at night. I stood with him on his left side, so he took my hand and placed me on his right side. Then he prayed thirteen rak'ahs. I estimated the length of his standing in each rak'ah to be the length of "O you who wraps himself [in garments]" (Surah al-Muzzammil).