Musnad Ahmad 6:10

'Abd al-Rahman ibn Mahdi narrated to us, Malik narrated to us, from his paternal uncle, from his father, that he heard Talhah ibn 'Ubayd Allah saying: A Bedouin came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "O Messenger of Allah, what is Islam?" He said: "Five prayers in a day and a night." He said: "Is there anything else upon me?" He said: "No." And he asked him about fasting, so he said: "Fasting of Ramadan." He said: "Is there anything else upon me?" He said: "No." He said: And he mentioned Zakat. He said: "Is there anything else upon me?" He said: "No." He said: "By Allah, I will not add to them, nor will I decrease from them." The Messenger of Allah, peace and blessings of Allah be upon him, said: "He has succeeded if he is truthful."