Sahih Muslim Book 4, Hadith Number 1675.

Chapter : Encouragement to observe prayers during Ramadhan and that is Tarawih.

Ibn ‘Abbas reported: I spent one night in the house of my mother’s sister Maimuna, daughter of Harith, and said to her: Awake me when the Messenger of Allah (may peace be upon him) stands to pray (at night). (She woke me up when) the Messenger of Allah (may peace be upon him) stood up for prayer. I stood on his left side. He took hold of my hand and made me stand on his right side, and whenever I dozed off he took hold of my earlobe (and made me alert). He (the narrator) said: He (the Holy Prophet) observed eleven rak’ahs. He then sat with his legs drawn and wrapped in his garment and slept so that I could bear his breathing while asleep. And when the dawn appeared, he observed two short rak’ahs of (Sunnah) prayer.