Sahih Muslim Book 6, Hadith Number 2484.
Chapter : The reward of one who does not observe fast because of a religious duty.
Anas (Allah be pleased with him) reported: We were with the Apostle of Allah (may peace be upon him) on a journey. Some of us had been observing the fast and some of us had not been fasting. We got down at a place on a hot day. Most of us had the cloth for shelter. There were also those amongst us who sheltered (themselves against the rays of the) sun with the help of their hands. The observers of the fast fell down (on account of weakness). Those who had not observed it got up and pitched tents and watered the mounts. Thereupon the Messenger of Allah (may peace be upon him) said: The breakers of the fast have taken away the reward today.