Sunan At-Tirmidhi

Narrated Samurah bin Jundab: That the Messenger of Allah (ﷺ) said: ""Kill the elder men among the idolaters and spare the Sharkh amont them."" And the Sharkh are the boys who did not begin to grow pubic hair. [Abu 'Eisa said:] This Hadith is Hasan Sahih. Hajjaj bin Artah narrated similarly from Qatadah