Musnad Abu Ya'la al-Mawsili 118:84

Humaid narrated to us, Shababa bin Sawwar narrated to us, 'Aqil narrated to us, from Ibn Shihab, from Anas, that the Prophet (may Allah bless him and grant him peace), when he wanted to combine the Dhuhr and 'Asr prayers while traveling, would delay the Dhuhr prayer until the time for 'Asr entered, then he would combine them.