Musnad Ahmad 1018:16

Waki’ narrated to us, Qurrah narrated to us, from Yazid ibn ‘Abdullah ibn al-Shikhkhir, who said: We were at this al-Mirbad in Basra. He said: A Bedouin came with a piece of leather, or a piece of a water skin, and said: "This is a document the Prophet, may Allah bless him and grant him peace, wrote for me." Abu al-‘Ala’ said: So I took it and read it to the people, and therein it said: "In the name of Allah, the Most Gracious, the Most Merciful. This is a document from Muhammad, the Messenger of Allah, may Allah bless him and grant him peace, to the Banu Zuhayr ibn Uqaysh: If you establish prayer, pay the zakat, and give from the spoils of war the fifth, the share of the Prophet, and the *safiyy* (the choice portion), then you are secure with the security of Allah and the security of His Messenger." He said: We said: "What did you hear from the Messenger of Allah, may Allah bless him and grant him peace?" He said: I heard him say: "Fasting the month of patience (Ramadan) and three days of every month removes the malice of the chest."