Sahih Bukhari
Shortening the Prayers (At-Taqseer)
Volume: 002 / Book Number: 020 / Hadith Number: 212
Narrated Anas bin Malik:
Whenever the Prophet started a journey before noon, he used to delay the Zuhr prayer till the time of 'Asr and then offer them together; and if the sun declined (at noon) he used to offer the Zuhr prayer and then ride (for the journey).