Sahih Al Bukhari Hadith Number 1110

Narrated Anas bin Malik: Whenever the Prophet (peace be upon him) 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).

18  Shortening the Prayers (At-Taqseer)
Hadith number 1110
Volume no 2
Chapter: 14 (Should the Adhan and Iqama be pronounced when the Maghrib and Isha prayers are offered together)