Ikrimah narrates from Ibn Abbas (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) combined Zuhr and Asr on a journey; when he (peace and blessings be upon him) was travelling on his mount, he would also combine Maghrib and Isha.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5532