This excerpt is taken from Sheikh Zubair Ali Zai Rahimahullah's Book on the Rules and Issues of Qurbani.
If the sacrificial animal falls ill:
It is narrated from Najiya Al-Khuza'i رضي الله عنه, the caretaker of the Prophet Muhammad's صلى الله عليه وسلم sacrificial camels, that I said: O Messenger of Allah! If one of the sacrificial camels falls ill and is unable to walk, what should I do? He صلى الله عليه وسلم said: "Slaughter it, then dip its foot in its blood and mark its chest with it, then leave it for the people."
Reference: (Musnad Al-Humaydi: 882)
Verification:
The chain is authentic.
Source:
Reference: It is found in Abu Dawood (1762), Turmidhi (910) who said: The hadith is Hasan Sahih, and Al-Hakim 447/1 authenticated it according to the conditions of Al-Bukhari and Muslim, and Al-Dhahabi agreed with him. We have also narrated it.