The Book on Government (Kitab Al-Imara)
Muslim :: Book 20 : Hadith 4633
Another version of the tradition narrated through a different chain of transmitters on the authority of Abu Huraira has the same wording as the previous tradition:" Allah takes care of one who goes out in the way of Allah" but ends in the words:" I would not lag behind any expedition which is undertaken to fight in the way of Allah, the Exalted."