Al-Harith narrates from Sayyiduna Ali (may Allah be pleased with him) that regarding the slaughter of the fetus the ruling is: "The slaughter of its mother is the slaughter of the fetus."
This has been narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) through several chains as marfu' (attributed to the Prophet), but its being marfu' is weak; the correct version is that it is mawquf (his own statement).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19495