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. (ID)
English reference (ID): Book 12, Hadith 1383 (ID)
Arabic reference (ID): Book 12, Hadith 1343 (ID)