Voluntary Fasting - Section 3 Mishkat al-Masabih 2074, 2075
Translation
He reported God’s messenger as saying, “If anyone out of a desire to please God fasts for a day, God will remove him from jahannam as far as a crow flies from the time it is a young bird till it dies of old age.”Ahmad transmitted it, and Baihaqi transmitted it in Shu'ab al-iman from Salama b. Qais.