Sahih Al Bukhari Hadith Number 1170

Narrated Jabir bin `Abdullah: While delivering a sermon, Allah’s Messenger (peace be upon him) said, “If anyone of you comes while the Imam is delivering the sermon or has come out for it, he should offer a two rak`at prayer.”

19  Prayer at Night (Tahajjud)
Hadith number 1170
Volume no 2
Chapter: 28 (What is recited in the two Rak’a (Sunna) of the Fajr)