When the Messenger of Allah ﷺ set out on a journey before the sun had passed its zenith, he would delay the Dhuhr prayer until the time of Asr, then dismount and combine the two prayers together. But if the sun passed its zenith before he set out, he would pray Dhuhr first and then mount his ride. ﷺ
حدثنا قتيبة وابن موهب المعنى قالا حدثنا المفضل عن عقيل عن ابن شهاب
كان رسول الله ﷺ إذا ارتحل قبل أن تزيغ الشمس أخر الظهر إلى وقت العصر ثم نزل فجمع بينهما فإن زاغت الشمس قبل أن يرتحل صلى الظهر ثم ركب ﷺ
Bukhari narrated it in Taqsir, Chapter, "If he set out after the sun had passed its zenith, he would pray Dhuhr, then mount his ride," Hadith 1112, and Muslim in Prayer of the Travelers, Chapter on the permissibility of combining the two prayers while traveling, Hadith 704, both of them from Qutayba, with this same chain.