Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates from the Messenger of Allah (peace and blessings be upon him): "Whoever sends off a voluntary sacrificial animal and it is lost, he may replace it if he wishes, or leave it if he wishes. If that sacrificial animal was from a vow, he must replace it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 10258