Ikrimah narrates from Ibn Abbas (may Allah be pleased with them both) that when the sun passed its zenith while the Messenger of Allah (peace and blessings be upon him) was at his house or halting place, he would combine Zuhr and Asr; and when the sun had not passed its zenith, he would travel on, and when the time of Asr came in, he would dismount and combine Zuhr and Asr. When the sun set while he (peace and blessings be upon him) was at his halting place, he would combine Maghrib and Isha; and when the sun had not set, he would set off, and when the time of Isha came, he would dismount and then combine Maghrib and Isha.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5530