Sunan An-Nasai, Chapter: The Book of Salah

from his father, that he heard Talhah bin Ubaidullah say: "A man from the people of Najd came to the Messenger of Allah (saw) with unkempt hair. We could hear him talking loudly but we could not understand what he was saying until he came closer. He was asking about Islam. The Messenger of Allah (saw) said to him: 'Five prayers each day and night.' He said: 'Do I have to do anything else?' He said: 'No, unless you do it voluntarily.' He said: 'And fasting the month of Ramadan.' He said: 'Do I have to do anything else?' He said: 'No, unless you do it voluntarily.' And the Messenger of Allah (saw) mentioned Zakah to him, and he said: 'Do I have to do anything else?' he said: 'No, unless you do it voluntarily.' The man left saying: 'By Allah, I will not do any more than this or any less.' The Messenger of Allah (saw) said: 'He will achieve salvation, if he is speaking the truth.'" (Sahih)