Sahih Muslim Book 4, Hadith Number 1579.

Chapter : Excellence observing two Rak’ahs of Sunnah in the dawn prayer.

Umm Habiba (the wife of the Holy Prophet) reported Allah’s Messenger (may peace be upon him) as saying: A house will be built in Paradise, for anyone who prays in a day and a night twelve rak’ahs; and she added: I have never abandoned (observing them) since I heard it from the Messenger of Allah (may peace be upon him). Some of the other narrators said the same words: I have never abandoned (observing them) since I heard (from so and so).