Sahih Bukhari Volume 4, Book 52, Hadith Number 200.

Narated By Anas : The Prophet offered a four-Rak’at Zuhr prayer at Medina and then offered a two Rak’at ‘Asr prayer at Dhul-Hulaifa and I heard the companions of the Prophet reciting Talbiya aloud (for Hajj and ‘Umra) altogether.