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.”
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا أَحْمَدُ بْنُ سَعِيدٍ الدَّارِمِيُّ، حَدَّثَنَا أَبُو عَاصِمٍ، أَخْبَرَنِي ابْنُ جُرَيْجٍ، حَدَّثَنِي عَمْرُو بْنُ دِينَارٍ، أَنَّهُ سَمِعَ طَاوُسًا، عَنِ ابْنِ عَبَّاسٍ، عَنْ عُمَرَ بْنِ الْخَطَّابِ، أَنَّهُ نَشَدَ النَّاسَ قَضَاءَ النَّبِيِّ صلى الله عليه وسلم فِي ذَلِكَ يَعْنِي فِي الْجَنِينِ فَقَامَ حَمَلُ بْنُ مَالِكِ بْنِ النَّابِغَةِ فَقَالَ كُنْتُ بَيْنَ امْرَأَتَيْنِ لِي فَضَرَبَتْ إِحْدَاهُمَا الأُخْرَى بِمِسْطَحٍ فَقَتَلَتْهَا وَقَتَلَتْ جَنِينَهَا فَقَضَى رَسُولُ اللَّهِ صلى الله عليه وسلم فِي الْجَنِينِ بِغُرَّةٍ عَبْدٍ وَأَنْ تُقْتَلَ بِهَا .
sunnah.com