Sayyiduna Utbah bin Abd (may Allah be pleased with him) narrated: The Prophet (peace and blessings be upon him) said: "Whoever keeps an obligatory fast for one day in the path of Allah, Allah Almighty removes Hell from him as far as the distance between the heavens and the seven earths; and whoever keeps a voluntary fast for one day, Allah Almighty removes Hell from him as far as the distance between the heaven and the earth."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir. Its chain contains a narrator, al-Waqidi, about whom much has been said, although he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصيام / 5177