Shortening the Prayers (At-Taqseer)

Bukhari :: Book 2 :: Volume 20 :: Hadith 213

Narrated Anas bin Malik:

Whenever the Prophet started the journey before noon, he used to delay the Zuhr prayer till the time for the 'Asr prayer and then he would dismount and pray them together; and whenever the sun declined before he started the journey he used to offer the Zuhr prayer and then ride (for the journey).