Sahih Al Bukhari Hadith Number 1163

Narrated `Aisha: The Prophet (peace be upon him) was never more regular and particular in offering any Nawafil than the two rak`at (Sunna) of the Fajr prayer.

19  Prayer at Night (Tahajjud)
Hadith number 1163
Volume no 2
Chapter: 26 (To talk after offering two Rak’a (Sunna of the Fajr prayer)