Sayyiduna Utbah bin Abd al-Sulami (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "Whoever keeps an obligatory fast in the path of Allah, Allah will distance Hell from him as far as the distance between the heavens and the seven earths; and whoever keeps a voluntary fast, Allah will distance Hell from him by the like of the distance of the sky."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14367