Nafi' narrates from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that whoever sends off a sacrificial animal (hady), and it is then lost or dies: if it was from a vow, he must replace it; if it was a voluntary offering, he may replace it if he wishes, or otherwise leave it. This is authentic as a mawquf report (a statement of the Companion).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10256