Musnad Ahmad 5:797

Hajjaj narrated to us, Shareek narrated to us, from ‘Asim ibn Kulayb, from Muhammad ibn Ka’b al-Qurazi: That ‘Ali, may Allah be pleased with him, said: "I have seen myself with the Messenger of Allah, may Allah bless him and grant him peace, and I would tie a stone to my stomach out of hunger, while my charity today is forty thousand."