It is narrated from Sayyiduna Aslam bin Bajrah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) put him in charge of the prisoners of Banu Qurayzah. He would look at a boy's private parts: if he saw that hair had grown, he would strike his neck, and whoever had not grown hair, he would take him and add him to the spoils of war of the Muslims.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 996