Sayyiduna Abu Dharr al-Ghifari (may Allah be pleased with him) narrates that he submitted: "O Messenger of Allah! What is fasting?" The Prophet (peace and blessings be upon him) said: "An obligation that suffices (and is rewarded)."
Ahmad mentioned it within a long hadith, which will come later in full, if Allah wills. Its chain contains a narrator who has not been named.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5083