It was narrated from 'Umar bin Khattab that :he asked the people about the ruling of the Prophet (ﷺ) concerning that - concerning a fetus. Hamal bin Malik bin Nabighah stood up and said: “I was between my two wives and one of them struck the other with a tent-pole, killing her and a fetus. The Messenger of Allah (ﷺ) ruled that the blood money for the fetus was a slave, and that she would be killed in retaliation.”

English translation · Sunan Ibn Majah