Sayyiduna Shaddad bin Aws (may Allah be pleased with him) said: I memorized two things from the Messenger of Allah (peace and blessings be upon him). He (peace be upon him) said: "Allah has prescribed excellence (i.e., mercy and justice) in everything. So when you kill (in retribution or jihad), do so quickly (do not prolong the killing), and when you slaughter an animal, slaughter it well. Let each one of you sharpen his knife and give comfort to his sacrificial animal (by slaughtering it swiftly), and do not subject it to torment."
Hadith Referenceسنن دارمي / من كتاب الاضاحي / 2009
(Commentary on Hadith 2008) The meaning of bringing comfort to the sacrificial animal is that after slaughtering, one should allow it to cool down (i.e., let the soul depart peacefully), and one should not subject it to suffering by slaughtering with a blunt knife. This is the system of mercy in Islam, that in every action, it teaches excellence and avoidance of harm. Even in the slaughtering of animals, this great principle should be put into practice, and the animal should not be killed by causing it to writhe in agony.