Sayyiduna Anas bin Malik (may Allah be pleased with him) said that when the Messenger of Allah (peace and blessings be upon him) set off before the sun had passed its zenith, he would delay Zuhr until the time of Asr, then dismount and combine the two; and if the sun passed its zenith before he set off, he would pray and then mount.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5520