From Sahih Muslim (Hadith 2623):
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
Reference: Muslim 2623