Musannaf Ibn Abi Shayba 28:879

Waki' narrated to us, he said: Al-A'mash narrated to us, from Abu Wa'il, who said: A letter from Umar came to us while we were in Khanaqin: "If you besiege a fortress and they want you to let them descend upon the judgment of Allah, then do not let them descend [upon that], for you do not know whether you will hit His judgment regarding them or not. Rather, let them descend upon your own judgment, then decide regarding them afterwards whatever you wish."