Sayyiduna Abu al-Tufayl narrates from Sayyiduna Mu'adh bin Jabal (may Allah be pleased with them both) that during the expedition of Tabuk, when the sun passed its zenith before the Messenger of Allah (peace and blessings be upon him) set off, he (peace and blessings be upon him) would combine Zuhr and Asr. If he set off before the sun passed its zenith, he would delay Zuhr until he dismounted for Asr, and he did the same with Maghrib: if the sun set before he set off, he would combine Maghrib and Isha, and if the sun set after he had set off, he would delay Maghrib and combine it with Isha.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5527