Bulugh al-Maram 2 The Book of Prayer كتاب الصلاة 348

وَلِأَبِي نُعَيْمٍ فِي “مُسْتَخْرَجِ مُسْلِمٍ” : { كَانَ إِذَا كَانَ فِي سَفَرٍ, فَزَالَتْ اَلشَّمْسُ صَلَّى اَلظُّهْرَ وَالْعَصْرَ جَمِيعًا, ثُمَّ اِرْتَحَلَ }

Abu Nu’aim has in his Mustakhraj of Muslim, “While on a journey, if the sun declined he (صلى الله عليه وسلم) used to pray Zuhr and ‘Asr together and then continue his journey.”