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.'
‘Abdallah b. Busr, on the authority of his sister as-Samma’, reported God’s messenger as saying, “Do not fast on Saturday except during what has been made obligatory for you; and if one of you can get nothing but a grape skin or a piece of wood from a tree, he should chew it.”
Ahmad, Abu Dawud, Tirmidhi, Ibn Majah and Darimi transmitted it.
‘Amir b. Mas'ud reported God’s messenger as saying, “Fasting in winter is booty gained without trouble.”*
*Literally, 'the cold booty.' The idea is that in winter one does not suffer from hunger and thirst as in summer.
Ahmad and Tirmidhi transmitted it, the latter saying this is a mursal tradition.