Musannaf Ibn Abi Shayba 13:1079

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from Ubayd Allah ibn Umar, from Dawud ibn al-Husayn, he said: I witnessed a young boy before one of the judges of the people of Medina, who was called Salamah ibn Abd al-Rahman al-Makhzumi. He sent for al-Qasim and Salim and asked them about his testimony. They said: "If he has grown pubic hair, then permit his testimony."