Yahya related to me from Malik from Nafi that Abdullah ibn Umar said, "If someone dedicates an animal and then it goes astray or dies, he should provide a substitute, if it was for a vow. If, however, it was voluntary, then he can either provide a substitute for it or not, as he wishes."
أَنَّهُ قَالَ مَنْ أَهْدَى بَدَنَةً ثُمَّ ضَلَّتْ أَوْ مَاتَتْ فَإِنَّهَا إِنْ كَانَتْ نَذْرًا أَبْدَلَهَا وَإِنْ كَانَتْ تَطَوُّعًا فَإِنْ شَاءَ أَبْدَلَهَا وَإِنْ شَاءَ تَرَكَهَا