Tamim bin Huways al-Misri said: I bought a sheep at Mina as a sacrifice, and it went astray. I asked Ibn Abbas (may Allah be pleased with them both) about it, and he said: It does not harm you.
Imam al-Shafi'i (may Allah have mercy on him) said: But if he finds it after having made it obligatory, he should slaughter it, even if the days of sacrifice have passed, just as is done with the sacrificial camels of the hady.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19197