Al-Mughirah bin Hadhaf al-Absi heard a man from Hamdan ask Sayyiduna Ali (may Allah be pleased with him) about a man who had bought a cow to sacrifice, and it then gave birth. Sayyiduna Ali (may Allah be pleased with him) said: Do not drink its milk except the surplus (that is, what is left over after the calf); and when the day of sacrifice comes, slaughter the cow and its calf on behalf of seven people.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10210
Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that he used to say: When a sacrificial camel gives birth, its young should be carried (on a mount) until it is slaughtered along with it; and if no mount can be found on which to carry it, then it should be carried on its mother until it is slaughtered along with her.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10211
The father of Hisham bin Urwah (may Allah have mercy on him) says: When you are compelled to ride your sacrificial camel, ride it without overburdening it; when you are compelled to (drink) its milk, drink what remains after its calf has had its fill; and when you slaughter it, slaughter its calf along with it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10212