Al-Matalib al-'Aliyah 3465 — What happened among the Children

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he said: "A man killed ninety-nine people, then he intended to repent. So he came to a monk who lived in a barren, desolate land and said: O monk, I have killed ninety-nine people; is there any room for repentance for me? He replied: No. The man said: Then the truth is that, by Allah, I will kill you and complete the number to a hundred. Then he came to another learned monk and said: I had taken ninety-nine lives and then completed the count of a hundred by killing a monk; is there any way of repentance for me now? He said: You have certainly transgressed greatly against yourself and committed a very grave sin, but remember that whoever repents, Allah Almighty accepts his repentance. The narrator said: The man at once threw down his sword and said: By Allah, I will now serve you until death separates us. The narrator said: He made a covenant with the monk that he would never disobey him. Then some travellers or famine-stricken people came there, and the monk used to practise medicine and treat people. The man asked the monk: Do you have any task for me? The monk said: Go and fire up the oven. So he went and heated the oven until it was blazing hot. Then he came and said: The oven has become hot; what is your command for me now? The monk said: Go and jump into it. So he went and leapt into the oven. Then the monk suddenly remembered him, so he and the people with him got up and went there, and they saw that the man was inside the oven drenched in sweat, while the fire had not harmed him in the least. Then the monk said: Now I know for certain that your repentance has been accepted in the court of Allah, so from now on I will serve you forever until you part from me."
Its chain is sahih (authentic), and there is a supporting narration (shahid) for it in Sahih al-Bukhari and Muslim from the hadith of Sayyiduna Abu Hurayrah (may Allah be pleased with him).