Riyad us-Salihin Hadith 1207

A person with dishevelled hair, one of the people of Najd, came to the Messenger of Allah (ﷺ). We heard the humming of his voice but could not fully understand what he was saying, till he approached close to the Messenger of Allah (ﷺ). Then I came to know that he was asking about Islam. The Messenger of Allah (ﷺ) said: "There are five (obligatory) Salat during the day and the night." He said: "Am I obliged to perform any other (Salat) besides these?" The Messenger of Allah (ﷺ) said, "No, but whatever you observe voluntarily." He (ﷺ) added, "There is the Saum of Ramadan." The inquirer asked: "Am I obliged to do anything besides this?" The Messenger of Allah (ﷺ) said, "No, but whatever you do out of your own free will. You may observe voluntary fasting." And the Messenger of Allah (ﷺ) told him about the Zakat (obligatory charity). The inquirer asked: "Am I obliged to pay anything besides this?" The Messenger of Allah (ﷺ) said, "No, but whatever you pay voluntarily out of your own free will." That man turned back saying: "By Allah! I will neither make any addition to this nor will I decrease anything from it." (Upon hearing this) the Messenger of Allah (ﷺ) remarked, "He is successful if he proves truthful (to what he is saying)." [Al-Bukhari and Muslim].