Musnad Ahmad 1018:101

Bahz narrated to us, Hammad narrated to us, Abu Ja'far al-Khatmi informed me, from Muhammad ibn Ka'b al-Qurazi, from Kathir ibn al-Sa'ib, who said: The sons of Qurayza narrated to me that they were presented to the Messenger of Allah, may Allah bless him and grant him peace, at the time of [the battle of] Qurayza. Whoever had grown pubic hair was killed, and whoever had not was left.