Sahih al-Bukhari Book 87, Hadith 11
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.
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."
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.