Musannaf Ibn Abi Shayba 28:1137

Waki’ narrated to us, he said: Al-A’mash narrated to us, from Khaythama, from Suwayd ibn Ghafala, who said: Ali said: "If I speak to you regarding what is between me and you, then war is deception. But if I speak to you about the Messenger of Allah, may Allah bless him and grant him peace, then for me to fall from the sky is more beloved to me than that I should lie about him."