Sahih Bukhari Volume 2, Book 21, Hadith Number 256.

Narated By ‘Aisha : Allah’s Apostle offered the ‘Isha’ prayer (and then got up at the Tahajjud time) and offered eight Rakat and then offered two Rakat while sitting. He then offered two Rakat in between the Adhan and Iqama (of the Fajr prayer) and he never missed them.