A man said, "O Allah's Messenger (ﷺ)! Which sin is the greatest in Allah's Sight?" The Prophet (ﷺ) said, "To set up a rival unto Allah though He Alone created you . " The man said, "What is next?" The Prophet (ﷺ) said, "To kill your son lest he should share your food with you." The man said, "What is next?" The Prophet said, "To commit illegal sexual intercourse with the wife of your neighbor." So Allah revealed in confirmation of this narration:-- 'And those who invoke not with Allah, any other god. Nor kill, such life as Allah has forbidden except for just cause nor commit illegal sexual intercourse. And whoever does this shall receive the punishment.' (25:68)

Allah's Messenger (ﷺ) said, "A faithful believer remains at liberty regarding his religion unless he kills somebody unlawfully."

One of the evil deeds with bad consequence from which there is no escape for the one who is involved in it is to kill someone unlawfully.

The Prophet (ﷺ) said, "The first cases to be decided among the people (on the Day of Resurrection) will be those of blood-shed."

An ally of Bani Zuhra who took part in the battle of Badr with the Prophet, that he said, "O Allah's Apostle! If I meet an unbeliever and we have a fight, and he strikes my hand with the sword and cuts it off, and then takes refuge from me under a tree, and says, 'I have surrendered to Allah (i.e. embraced Islam),' may I kill him after he has said so?" Allah's Messenger (ﷺ) said, "Do not kill him." Al- Miqdad said, "But O Allah's Messenger (ﷺ)! He had chopped off one of my hands and he said that after he had cut it off. May I kill him?" The Prophet (ﷺ) said. "Do not kill him for if you kill him, he would be in the position in which you had been before you kill him, and you would be in the position in which he was before he said the sentence." The Prophet (ﷺ) also said to Al-Miqdad, "If a faithful believer conceals his faith (Islam) from the disbelievers, and then when he declares his Islam, you kill him, (you will be sinful). Remember that you were also concealing your faith (Islam) at Mecca before."

The Prophet (ﷺ) said, "No human being is killed unjustly, but a part of responsibility for the crime is laid on the first son of Adam who invented the tradition of killing (murdering) on the earth. (It is said that he was Qabil).

The Prophet (ﷺ) said, "After me (i.e. after my death), do not become disbelievers, by striking (cutting) the necks of one another.

The Prophet (ﷺ) said during Hajjat-al-Wada`, "Let the people be quiet and listen to me. After me, do not become disbelievers, by striking (cutting) the necks of one another."

The Prophet (ﷺ) said, "Al-Ka`ba'ir (the biggest sins) are: To join others (as partners) in worship with Allah, to be undutiful to one's parents," or said, "to take a false oath." (The sub-narrator, Shu`ba is not sure) Mu`adh said: Shu`ba said, "Al-Ka`ba'ir (the biggest sins) are: (1) Joining others as partners in worship with Allah, (2) to take a false oath (3) and to be undutiful to one's parents," or said, "to murder (someone unlawfully).

The Prophet (ﷺ) said, "The biggest of Al-Ka`ba'ir (the great sins) are (1) to join others as partners in worship with Allah, (2) to murder a human being, (3) to be undutiful to one's parents (4) and to make a false statement," or said, "to give a false witness."

Allah's Messenger (ﷺ) sent us (to fight) against Al-Huraqa (one of the sub-tribes) of Juhaina. We reached those people in the morning and defeated them. A man from the Ansar and I chased one of their men and when we attacked him, he said, "None has the right to be worshipped but Allah." The Ansari refrained from killing him but I stabbed him with my spear till I killed him. When we reached (Medina), this news reached the Prophet. He said to me, "O Usama! You killed him after he had said, 'None has the right to be worshipped but Allah?"' I said, "O Allah's Messenger (ﷺ)! He said so in order to save himself." The Prophet (ﷺ) said, "You killed him after he had said, 'None has the right to be worshipped but Allah." The Prophet (ﷺ) kept on repeating that statement till I wished I had not been a Muslim before that day.

I was among those Naqibs (selected leaders) who gave the Pledge of allegiance to Allah's Messenger (ﷺ). We gave the oath of allegiance, that we would not join partners in worship besides Allah, would not steal, would not commit illegal sexual intercourse, would not kill a life which Allah has forbidden, would not commit robbery, would not disobey (Allah and His Apostle), and if we fulfilled this pledge we would have Paradise, but if we committed any one of these (sins), then our case will be decided by Allah.

The Prophet (ﷺ) said, "Whoever carries arms against us, is not from us."

I went to help that man (i.e., `Ali), and on the way I met Abu Bakra who asked me, "Where are you going?" I replied, "I am going to help that man." He said, "Go back, for I heard Allah's Messenger (ﷺ) saying, 'If two Muslims meet each other with their swords then (both) the killer and the killed one are in the (Hell) Fire.' I said, 'O Allah's Messenger (ﷺ)! It is alright for the killer, but what about the killed one?' He said, 'The killed one was eager to kill his opponent."

A Jew crushed the head of a girl between two stones, and the girl was asked, "Who has done that to you, so-and-so or so and so?" (Some names were mentioned for her) till the name of that Jew was mentioned (whereupon she agreed). The Jew was brought to the Prophet (ﷺ) and the Prophet (ﷺ) kept on questioning him till he confessed, whereupon his head was crushed with stones.