كتاب الصيام
13
The Book of Fasting
(17)
Chapter: The choice between fasting and not fasting when travelling
(17)
باب التَّخْيِيرِ فِي الصَّوْمِ وَالْفِطْرِ فِي السَّفَرِ ‏‏
Sahih Muslim 1122 b

Abu Darda' reported:

We were with the Messenger of Allah (ﷺ) on some of his journeys on an intensely hot day so much so that a person would place his hand on his head (in order to protect himself) against excessive heat, and none amongst us was fasting but the Messenger of Allah (ﷺ) and Abdullah b. Rawaha
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مَسْلَمَةَ الْقَعْنَبِيُّ، حَدَّثَنَا هِشَامُ بْنُ سَعْدٍ، عَنْ عُثْمَانَ بْنِ حَيَّانَ، الدِّمَشْقِيِّ عَنْ أُمِّ الدَّرْدَاءِ، قَالَتْ قَالَ أَبُو الدَّرْدَاءِ لَقَدْ رَأَيْتُنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي بَعْضِ أَسْفَارِهِ فِي يَوْمٍ شَدِيدِ الْحَرِّ حَتَّى إِنَّ الرَّجُلَ لَيَضَعُ يَدَهُ عَلَى رَأْسِهِ مِنْ شِدَّةِ الْحَرِّ وَمَا مِنَّا أَحَدٌ صَائِمٌ إِلاَّ رَسُولُ اللَّهِ صلى الله عليه وسلم وَعَبْدُ اللَّهِ بْنُ رَوَاحَةَ ‏.‏
Reference : Sahih Muslim 1122 b
In-book reference : Book 13, Hadith 137
USC-MSA web (English) reference : Book 6, Hadith 2493
  (deprecated numbering scheme)