كتاب الصيام
13  
The Book of Fasting
(16)
Chapter: The reward of the one who does not fast when travelling if he does any tasks that are required
(16)
باب أَجْرِ الْمُفْطِرِ فِي السَّفَرِ إِذَا تَوَلَّى الْعَمَلَ ‏‏
Sahih Muslim 1119 b

Anas (Allah be pleased with him) reported that the Messenger of Allah (ﷺ) was journeying (along with his Companions). Some of them had observed the fast whereas the others had broken it. Those who did not fast girded up their loins and worked, but the observers of the fast were too weak to work. Upon this he (the Messenger of Allah) said:

Today the breakers of the fast have gone with the reward.
وَحَدَّثَنَا أَبُو كُرَيْبٍ، حَدَّثَنَا حَفْصٌ، عَنْ عَاصِمٍ الأَحْوَلِ، عَنْ مُوَرِّقٍ، عَنْ أَنَسٍ، - رضى الله عنه - قَالَ كَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم فِي سَفَرٍ فَصَامَ بَعْضٌ وَأَفْطَرَ بَعْضٌ فَتَحَزَّمَ الْمُفْطِرُونَ وَعَمِلُوا وَضَعُفَ الصُّوَّامُ عَنْ بَعْضِ الْعَمَلِ - قَالَ - فَقَالَ فِي ذَلِكَ ‏ "‏ ذَهَبَ الْمُفْطِرُونَ الْيَوْمَ بِالأَجْرِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1119b
In-book reference : Book 13, Hadith 129
USC-MSA web (English) reference : Book 6, Hadith 2485
  (deprecated numbering scheme)