Arabic text
عَنْ أَبِي يَعْلَى شَدَّادِ بْنِ أَوْسٍ رَضِيَ اللَّهُ عَنْهُ عَنْ رَسُولِ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ: «إِنَّ اللهَ كَتَبَ الْإِحْسَانَ عَلَى كُلِّ شَيْءٍ، فَإِذَا قَتَلْتُمْ فَأَحْسِنُوا القِتْلَةَ، وَإِذَا ذَبَحْتُمْ فَأَحْسِنُوا الذِّبْحَةَ، وَلْيُحِدَّ أَحَدُكُمْ شَفْرَتَهُ، وَلْيُرِحْ ذَبِيحَتَهُ».
Translation
Abu Ya‘la Shaddād ibn Aws (may Allah be pleased with him) reported that the Messenger of Allah (ﷺ) said: "Indeed, Allah has enjoined Ihsān (kindness/excellence) in all things. So, when you kill, kill with kindness; and when you slaughter, slaughter with kindness. Let one of you sharpen his blade and spare the animal he slaughters suffering."
Explanation
The Prophet (may Allah's peace and blessings be upon him) informs that Allah Almighty prescribed ihsān (kindness/excellence) upon us in everything. Ihsān: It is constantly being heedful of Allah in worshipping Him, doing good, and abstaining from harming other created beings. An aspect of this is ihsān in killing and slaughtering. Ihsān in killing, for legal retribution, is to choose the easiest method which causes the least suffering to the one being killed and the quickest in taking his life. Ihsān in slaughtering animals is to show kindness toward the animal by sharpening the blade and not sharpening it while the animal is looking on, and not slaughtering an animal while another animal is looking.
Source: HadeethEnc.com