Muwatta Malik 858

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّهُ قَالَ مَنْ أَهْدَى بَدَنَةً ثُمَّ ضَلَّتْ أَوْ مَاتَتْ فَإِنَّهَا إِنْ كَانَتْ نَذْرًا أَبْدَلَهَا وَإِنْ كَانَتْ تَطَوُّعًا فَإِنْ شَاءَ أَبْدَلَهَا وَإِنْ شَاءَ تَرَكَهَا ‏.‏ وَحَدَّثَنِي عَنْ مَالِكٍ أَنَّهُ سَمِعَ أَهْلَ الْعِلْمِ يَقُولُونَ لاَ يَأْكُلُ صَاحِبُ الْهَدْىِ مِنَ الْجَزَاءِ وَالنُّسُكِ ‏.‏

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

Reference

Muwatta Malik 858

In-book

Book 20, Hadith 154