Musannaf Ibn Abi Shayba 18:33

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Ibn Abi Najih, from Mujahid, from Umar, that he said regarding quasi-intentional (shibh al-‘amd) [homicide]: Thirty jadha’ah (four-year-old camels) and thirty hiqqah (three-year-old camels) and forty [camels] between thaniyyah (two-year-old) and bazil (six-year-old) in their year, all of them being khalifah (pregnant).