Muwatta Malik, Book 20, Hadith 157 (USC-MSA)

Yahya related to me from Malik that Ibn Shihab said, "If someone dedicates an animal as compensation, or for a vow, or as the sacrifice for tamattu, and misfortune befalls it on the road, he must provide a substitute."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنِ ابْنِ شِهَابٍ، أَنَّهُ قَالَ مَنْ أَهْدَى بَدَنَةً جَزَاءً أَوْ نَذْرًا أَوْ هَدْىَ تَمَتُّعٍ فَأُصِيبَتْ فِي الطَّرِيقِ فَعَلَيْهِ الْبَدَلُ ‏.‏

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also referenced under: Book 20, Hadith 860 (In-Book)