Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 195
Narrated Anas bin Malik :
offered four Rakat of Zuhr prayer with the Prophet (p.b.u.h) at Medina and two Rakat at Dhul-Hulaifa. (i.e. shortened the 'Asr prayer).