Sayyiduna Abu Layla (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) was asked about slaughtering the young in the womb of an animal, and he said: "The slaughter of its mother counts as its slaughter."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains Halbas bin Muhammad, who is matruk (abandoned).