Sahih Muslim Book - 6, Hadith - 1625

وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ حَدَّثَنَا الْمُفَضَّلُ - يَعْنِى ابْنَ فَضَالَةَ - عَنْ عُقَيْلٍ عَنِ ابْنِ شِهَابٍ عَنْ أَنَسِ بْنِ مَالِكٍ قَالَ كَانَ رَسُولُ اللَّهِ -صلى الله عليه وسلم- إِذَا ارْتَحَلَ قَبْلَ أَنْ تَزِيغَ الشَّمْسُ أَخَّرَ الظُّهْرَ إِلَى وَقْتِ الْعَصْرِ ثُمَّ نَزَلَ فَجَمَعَ بَيْنَهُمَا فَإِنْ زَاغَتِ الشَّمْسُ قَبْلَ أَنْ يَرْتَحِلَ صَلَّى الظُّهْرَ ثُمَّ رَكِبَ.

It was narrated that Anas bin Malik said: "If the Messenger of Allah (s.a.w) was setting off before the sun had passed its zenith, he would delay Zuhr until the time of 'Asr began, then he would dismount and join them. If the sun had passed its zenith before he set out, he would pray Zuhr and then set out."