Ibn Shihab (may Allah have mercy on him) said: I asked Salim bin Abdullah (may Allah have mercy on him): May Zuhr and Asr be combined on a journey? He said: There is no harm in it. Have you not seen what the people do at Arafah?
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5541