Anas bin Malik (may Allah be pleased with him) (a man of Banu Ka'b) narrated: The cavalry of the Messenger of Allah (peace and blessings be upon him) raided us, so I reached him while he was eating. He (peace and blessings be upon him) said: "Sit down and have some of this food of ours." I said: O Messenger of Allah, I am fasting. He (peace and blessings be upon him) said: "Sit down, I will tell you about prayer and fasting: Allah Almighty has relieved the traveller of a portion - or half - of the prayer, and has relieved the traveller, the sick person and the pregnant woman of fasting (sawm) - or fasting (siyam)." By Allah, he (peace and blessings be upon him) said all of these, or one of them. Then I reproached myself for not having eaten from the food of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 761