Sayyiduna Anas bin Malik (may Allah be pleased with him) said that when the Messenger of Allah (peace and blessings be upon him) was on a journey and the sun passed its zenith, he would pray Zuhr and Asr together. Then he would set off.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5523