Musannaf Ibn Abi Shayba 28:986

Abu Dawud al-Tayalisi narrated to us, from Abu Hurrah, who said: Al-Hasan was asked about a people who are on a military expedition, and they capture a non-Arab (al-'ilj), and they force him to show them the weak points of the enemy. Al-Hasan said: "That used to be done."