Imam Abu Bakr (may Allah have mercy on him) says that in the hadith of Sayyiduna Talhah bin Ubaidullah (may Allah be pleased with him), in which a Bedouin asked the Prophet Muhammad (peace and blessings be upon him) about Islam, it is mentioned that the Prophet (peace and blessings be upon him) said: "And the fasting of Ramadan is obligatory." He asked, "Are there any other fasts obligatory upon me besides the fasts of Ramadan?" The Prophet (peace and blessings be upon him) said: "No, except that you observe voluntary fasts."