Sahih Al Bukhari Hadith Number 1161

Narrated `Aisha: After offering the Sunna of the Fajr prayer, the Prophet (peace be upon him) used to talk to me, if I happen to be awake; otherwise he would lie down till the Iqama call was proclaimed (for the Fajr prayer).

19  Prayer at Night (Tahajjud)
Hadith number 1161
Volume no 2
Chapter: 24 (Whoever talks after offering two Rak’a (Sunna) (of the Fajr prayer)