Nafi' (may Allah have mercy on him) said that Abdullah bin Umar (may Allah be pleased with them both) used to say: Any man who sends a sacrificial animal (hady) and it goes astray: if it was a vow, he must replace it; and if it was voluntary, then if he wishes he replaces it, and if he wishes he leaves it.
Abdullah bin Amir al-Aslami narrated it from Nafi' as marfu' (from the Prophet), but the correct version is mawquf (a statement of Ibn Umar).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19196