Sahih Al Bukhari Hadith Number 1162

Narrated `Aisha: After offering the two rak`at (Sunna) the Prophet (peace be upon him) used to talk to me, if I happen to be awake; otherwise he would lie down.

19  Prayer at Night (Tahajjud)
Hadith number 1162
Volume no 2
Chapter: 25 (What is said about the Nawafil being offered as two Rak’a followed by two and so on)