Sahih Muslim Book 001, Hadith Number 0007.

Chapter : On Prayer (Salat) which is one of the Pillars of Iman.

It is reported on the authority of Talha b. 'Ubaidullah that a person with dishevelled hair, one of the people of Nejd, came to the Messenger of Allah (may peace be upon him). We heard the humming of his voice but could not fully discern what he had been saying, till he came nigh to the Messenger of Allah (may peace be upon him). It was then (disclosed to us) that he was asking questions pertaining to Islam. The Messenger of Allah (may peace be upon him) said: Five prayers during the day and the night. (Upon this he said: Am I obliged to say any other (prayer) besides these? He (the Holy Prophet,) said: No, but whatever you observe voluntarily, out of your own free will, and the fasts of Ramadan. The inquirer said: Am I obliged to do anything else besides this? He (the Holy Prophet) said: No, but whatever you do out of your own free will. And the Messenger of Allah told him about the Zakat (poor-rate). The inquirer said: Am I obliged to pay anything else besides this? He (the Holy Prophet) said: No, but whatever you pay voluntarily out of your own free will. The man turned back and was saying: I would neither make any addition to this, nor will decrease anything out of it. The Prophet remarked: He is successful, if he is true to what he affirms.