Sayyiduna Mu'adh bin Anas (may Allah be pleased with him) narrated: The Prophet (peace and blessings be upon him) said: "Whoever fasts one day in the path of Allah, outside Ramadan, is removed from Hell by the distance of a hundred years, which a trained horse (covers in a hundred years)."
Imam Abu Ya'la recorded it. Its chain contains a narrator, Zabban bin Fa'id, about whom much has been said, although he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5171