Abu Sa'id Khudri (RAA) narrated that the Messenger of Allah (ﷺ) said:
"The (legal) slaughtering of the fetus is included when its mother is slaughtered." Related by Ahmad and Ibn Hibban graded it as Sahih. (IT)
English reference (IT): Book 12, Hadith 1383 (IT)
Arabic reference (IT): Book 12, Hadith 1343 (IT)