Sahih Muslim
The Book of Jihad and Expedition (Kitab Al-Jihad wal-Siyar)
Book Number: 019 / Hadith Number: 4407
The same tradition has been narrated through a different chain of transmitters on the authority of A'mash. This version contains the words: Ila amrin yofzi'una instead of Ila amrin na'rifuhu.