Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrated: I was in the house of my maternal aunt, Sayyidah Maymunah (may Allah be pleased with her), when the Prophet (peace and blessings be upon him) stood to pray at night. I stood on his left, so he (peace and blessings be upon him) took me by the hand and placed me on his right. Then he (peace and blessings be upon him) prayed thirteen rak'ahs. I estimated that in each rak'ah he (peace and blessings be upon him) stood for as long as (it takes to recite) Surah al-Muzzammil.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 11225