Sahih Al Bukhari Hadith Number 1112

Narrated Anas bin Malik: Whenever the Prophet (peace be upon him) started on a journey before noon, he used to delay the Zuhr prayers till the time for the ‘Asr prayer and then he would dismount and offer them together; and whenever the sun declined before he started on a journey he used to offer the Zuhr prayers and then ride (for journey).

18  Shortening the Prayers (At-Taqseer)
Hadith number 1112
Volume no 2
Chapter: 16 (Whenever a person travels after midday, he should offer the Zuhr prayers and the ride for the journey)