Maliks Muwatta Book 20, Hadith Number 144.

Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, “If a she-camel that is being driven as a sacrificial animal gives birth, the offspring should be carried along as well and they are sacrificed together with her, and if there is no place where they can be carried, they should be carried on the mother until they are all sacrificed.”