Sunnah.com reference : Book 2, Hadith 350English translation : Book 2, Hadith 433Arabic reference : Book 2, Hadith 438
Narrated Anas (RA):
When Allah's Messenger (ﷺ) (while on a journey) proceeded before the sun had declined from the centre of the sky, he (ﷺ) delayed the Zuhr prayer till the time of the 'Asr prayer; he (ﷺ) would then alight and combine the two prayers. If the sun declined before he (ﷺ) moved off, he (ﷺ) would offer the Zuhr prayer and ride (i.e. travel).

[Agreed upon].
وَعَنْ أَنَسٍ: { كَانَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-إِذَا اِرْتَحَلَ قَبْلَ أَنْ تَزِيغَ اَلشَّمْسُ أَخَّرَ اَلظُّهْرَ إِلَى وَقْتِ اَلْعَصْرِ, ثُمَّ نَزَلَ فَجَمَعَ بَيْنَهُمَا, فَإِنْ زَاغَتْ اَلشَّمْسُ قَبْلَ أَنْ يَرْتَحِلَ صَلَّى اَلظُّهْرَ, ثُمَّ رَكِبَ }   1‏ مُتَّفَقٌ عَلَيْهِ.‏‏1 ‏- صحيح.‏ رواه البخاري (2/582‏-583)‏، ومسلم (704)‏.‏