Musannaf Ibn Abi Shayba 18:446

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Hajjaj, from Makhul, from Zayd ibn Thabit: That he judged regarding the nipple of a woman’s breast to be one-fourth of her blood money (diyah), and regarding the nipple of a man’s breast to be one-eighth of his blood money.