Musannaf Ibn Abi Shayba 19:82

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Isma’il bin Umayyah, from Yahya, who said: A youth among us became infatuated with a woman in poetry, so he was brought to ‘Umar. He doubted whether he had reached puberty, and it was not found that he had grown pubic hair. He said: "If I had found that you had grown pubic hair, I would have flogged you or I would have applied the prescribed punishment (hadd) to you."