It is narrated from Sayyiduna Umar (may Allah be pleased with him) that (he said): Let the person stand who has heard anything from the Messenger of Allah (peace and blessings be upon him) concerning the fetus. So Haml bin Malik bin al-Nabighah (may Allah be pleased with him) stood and said: "I had two wives; one of them struck the other with a stick and her child miscarried, so he (peace and blessings be upon him) fixed 'a slave' as blood money for it." Sayyiduna Umar (may Allah be pleased with him) said: "Had this not been (heard), I would have given some other ruling about it today."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16409