Musannaf Ibn Abi Shayba 19:866

Abu Bakr narrated to us, he said: Al-Fadl ibn Dukayn narrated to us, from Aban ibn Abdullah, from Ibrahim ibn Jarir, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I have been commanded to fight the people until they say: 'There is no god but Allah.'"