"Until, when he reached the setting of the sun, he found it set in a spring of murky water: Near it he found a People: We said: “O Zul-qarnain! (thou hast authority,) either to punish them, or to treat them with kindness." - Quran 18:86 (read more)

"Narrated Atiyyah al-Qurazi: I was among the captives of Banu Qurayzah. They (the Companions) examined us, and those who had begun to grow hair (pubes) were killed, and those who had not were not killed. I was among those who had not grown hair." - Sunan Abu Dawud 38:4390 (read more)

"Apostasy is of two types: ordinary apostasy and extreme apostasy, for which execution is prescribed. In both cases there is evidence that it is essential to execute the apostate, but the evidence indicating that the sentence of death may be waived if the person repents does not apply to both types of apostasy. Rather the evidence indicates that that is allowed only in the first case – i.e., ordinary apostasy." - Shaykh al-Islam Ibn Taymiyah