Musnad Ahmad 32:1404

Ali ibn Ishaq and al-Hasan ibn Yahya narrated to us, they said: Abdullah—meaning Ibn al-Mubarak—narrated to us, he said: Humayd al-Tawil informed us, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) said: "I have been commanded to fight the people until they bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah. So if they bear witness that there is no god but Allah and that Muhammad is the Messenger of Allah, and they face our qibla, and 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."