Sayyiduna Talhah bin Ubaydullah (may Allah be pleased with him) said: 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 near the Prophet (peace and blessings be upon him), and he was asking about Islam. The Prophet (peace and blessings be upon him) said: "Five prayers in the day and night are obligatory." He said: Is anything else obligatory upon me besides them? He (peace and blessings be upon him) said: "No, unless you pray voluntarily." And (he (peace and blessings be upon him) said:) "The fasts of Ramadan are obligatory." He said: Is anything else obligatory upon me besides these fasts? He (peace and blessings be upon him) said: "No, unless you observe voluntary fasts." The Prophet (peace and blessings be upon him) mentioned zakah to him. He said: Is anything else obligatory upon me besides it? He (peace and blessings be upon him) said: "No, unless you give voluntary charity." The man turned away, saying: By Allah, I will neither add to this nor take anything away from it. The Prophet (peace and blessings be upon him) said: "He has succeeded if he has spoken the truth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4448