It is narrated from Sayyiduna Shaddad bin Aws (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, Allah has prescribed excellence (ihsan) in everything. So when you kill (in retribution or legal punishment), kill in the best manner, and when you slaughter an animal, slaughter it in the best manner. A person should sharpen his knife and give comfort to the animal being slaughtered."
Hadith Referenceالسنن المأثورة / باب في أكل لحوم الخيل والبغال والحمير / 577