Kurayb narrates from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Shall I not tell you about the prayer of the Messenger of Allah (peace and blessings be upon him) on a journey? We said: Certainly. He said: When the sun passed its zenith while he was at his halting place, he would combine Zuhr and Asr before mounting; and when it had not passed its zenith while he was at his halting place, he (peace and blessings be upon him) would travel on, and when the time of Asr came, he would dismount and combine Zuhr and Asr. When the time of Maghrib came while he was at his halting place, he would combine it with Isha; and when it had not come while he was at his halting place, he would mount, and when the time of Isha came, he would dismount and pray Maghrib and Isha both together.
It is strengthened by the supporting reports mentioned before.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5531