Musannaf Ibn Abi Shayba 7:437

Waki’ narrated to us, from Salam ibn Miskin, that he asked Jabir ibn Zayd and al-Hasan about a woman who made a sacrificial offering (hady) incumbent upon herself. Jabir ibn Zayd said: "If she is wealthy, then a cow; and if she is in straitened circumstances, then a sheep." Al-Hasan said: "It is an expiation for an oath; she fasts three days."