He said: 'No, unless you volunteer.'
The man departed saying: 'By Allah, I will not add to or decrease from this.'
The Messenger of Allah, peace be upon him, said: 'He will succeed if he is truthful.'

A man asks the Prophet about Islam's obligations. Told the essential prayers, fasting, and alms suffice, he vows to do only these and is promised success if truthful.
He said: 'No, unless you volunteer.'
The man departed saying: 'By Allah, I will not add to or decrease from this.'
The Messenger of Allah, peace be upon him, said: 'He will succeed if he is truthful.'