Musnad Ahmad 32:1113

Ali ibn Ishaq narrated to us, he said: Abdullah informed us, Humayd al-Tawil informed us, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have been commanded to fight the people until they testify that there is no god but Allah and that Muhammad is the Messenger of Allah. If they testify to that, face our qibla, eat our slaughtered animals, and pray our prayer, then their blood and their wealth have become forbidden to us, except by its right. They have what the Muslims have, and upon them is what is upon them."