Musannaf Ibn Abi Shayba 19:899

Abu Bakr narrated to us, he said: Yazid ibn Harun narrated to us, from Sufyan ibn Husayn, from al-Hasan, who said: If a man commits adultery with a young girl, he is to be flogged and not stoned, and there is nothing upon the young girl. And if a young boy commits adultery with a woman, she is to be flogged and not stoned, and upon the young boy is a discretionary punishment (ta'zir).