Voluntary Fasting - Section 2 Mishkat al-Masabih 2064
Translation
Abu Umama reported God’s messenger as saying, "If anyone fasts for a day while on jihad,* God will put between him and hell a ditch as wide as the distance between heaven and earth." *Mirqat gives this meaning, but says it may refer to the hajj, the 'umra, seeking knowledge, or desiring to please God. The Arabic is 'in God’s way.'Tirmidhi transmitted it.