Sahih Al Bukhari Hadith Number 1089

Narrated Anas bin Malik: offered four rak`at of Zuhr prayer with the Prophet (peace be upon him) at Medina and two rak`at at Dhul-Hulaifa. (i.e. shortened the `Asr prayer).

18  Shortening the Prayers (At-Taqseer)
Hadith number 1089
Volume no 2
Chapter: 5 (When a traveller leaves his original place, he can shorten his Salat (prayer)