Musnad Ahmad 937:4

Abu al-Yaman narrated to us, Isma'il ibn 'Ayyash narrated to us, from Ma'an ibn Rifa'ah, from Abu Khalaf, from Anas ibn Malik, from Abu Dharr, from the Prophet, may Allah bless him and grant him peace, that he said: "Islam is docile; it is not ridden except by one who is docile."