Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates from the Noble Prophet (peace and blessings be upon him) that he said: "Whoever sends a camel for a voluntary sacrifice and it gets lost on the way, then if he wishes, he may send a substitute for that sacrifice, and if he wishes, he may not send one. But if he was sending the sacrificial animal due to a vow, then he must definitely send a substitute."