The father of Abu Suhayl bin Malik heard Sayyiduna Talhah bin Ubaydullah (may Allah be pleased with him) say: A man from the people of Najd, with dishevelled hair, came to the Prophet (peace and blessings be upon him). We could hear the humming of his voice but could not understand what he was saying, until he came close to the Messenger of Allah (peace and blessings be upon him) and asked about Islam. The Messenger of Allah (peace and blessings be upon him) said: "Five prayers in the day and night." He said: Is there anything else upon me besides them? He (peace and blessings be upon him) said: "No, unless you pray voluntarily." Then the Messenger of Allah (peace and blessings be upon him) said: "And fasting the month of Ramadan." He said: Is there anything else upon me besides it? He (peace and blessings be upon him) said: "No, unless you fast voluntarily." The narrator says: The Messenger of Allah (peace and blessings be upon him) also mentioned zakah to him. He said: Is there anything else upon me besides it? He (peace and blessings be upon him) said: "Unless you give voluntary charity." The man turned away, saying: By Allah, I will neither do more than this nor less. The Messenger of Allah (peace and blessings be upon him) said: "He has succeeded, if he has spoken the truth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1691