Sahih Muslim 11a

Qutaybah ibn Sa'id ibn Jamil ibn Tarif ibn Abdullah al-Thaqafi narrated to us, from Malik ibn Anas—while it was being read to him—from Abu Suhayl, from his father, that he heard Talhah ibn Ubaydullah say: A man from the people of Najd, with disheveled hair, came to the Messenger of Allah (peace and blessings be upon him). We could hear the sound of his voice but could not understand what he was saying until he came close to the Messenger of Allah; then he was asking about Islam. The Messenger of Allah (peace and blessings be upon him) said, ‘Five prayers during the day and night.’ He asked, ‘Am I required to do anything beyond them?’ He said, ‘No, unless you do so voluntarily. And fasting the month of Ramadan.’ He asked, ‘Am I required to do anything beyond it?’ He said, ‘No, unless you do so voluntarily.’ The Messenger of Allah mentioned zakat to him. He asked, ‘Am I required to do anything beyond it?’ He said, ‘No, unless you do so voluntarily.’ The man turned away, saying, ‘By Allah, I will neither add anything to this nor reduce anything from it.’ The Messenger of Allah said, ‘He will prosper if he is truthful.’