Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 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."