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)
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."
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 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).