Sahih al-Bukhari Book 87, Hadith 10
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."
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).
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.