Hadith: Verily, Allah has enjoined kindness in all things

عن شداد بن أوس رضي الله عنه قال: ثِنْتَانِ حَفِظْتُهُمَا عَنْ رَسُولِ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ قَالَ:
«إِنَّ اللهَ كَتَبَ الْإِحْسَانَ عَلَى كُلِّ شَيْءٍ، فَإِذَا قَتَلْتُمْ فَأَحْسِنُوا الْقِتْلَةَ، وَإِذَا ذَبَحْتُمْ فَأَحْسِنُوا الذَّبْحَ، وَلْيُحِدَّ أَحَدُكُمْ شَفْرَتَهُ، فَلْيُرِحْ ذَبِيحَتَهُ».
[صحيح] - [رواه مسلم] - [صحيح مسلم: 1955]

Shaddād ibn Aws (may Allah be pleased with him) reported: There are two things which I learned by heart from the Messenger of Allah (may Allah’s peace and blessings be upon him); he said:
"Verily, Allah has enjoined kindness 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 from suffering."
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 1955]

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.