Musnad Ahmad 811:1

Isma'il narrated to us, Ayyub narrated to us, he said: Abu Qilabah had narrated this hadith to me, then he said to me: "Do you want the one who narrated it to me?" He said: So he directed me to him, and I came to him. He said: A relative of mine, who is called Anas ibn Malik, narrated to me, he said: I came to the Messenger of Allah, may Allah bless him and grant him peace, regarding some camels of a neighbor of mine that had been taken. I found him while he was eating, and he invited me to his food. I said: "I am fasting." He said: "Draw near," or he said: "Come, I will inform you about that. Indeed, Allah has relieved the traveler of fasting and half of the prayer, and [likewise] the pregnant woman and the nursing mother." He said: After that, he would regret it and say: "Would that I had eaten from the food of the Messenger of Allah, may Allah bless him and grant him peace, when he invited me to it."