Sahih Muslim Book 6, Hadith Number 2485.

Chapter : The reward of one who does not observe fast because of a religious duty.

Anas (Allah be pleased with him) reported that the Messenger of Allah (may peace be upon him) 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 Apostle of Allah) said: Today the breakers of the fast have gone with the reward.