From Sahih Bukhari (Hadith 6878):
Narrated `Abdullah:Allah's Messenger (ﷺ) said, "The blood of a Muslim who confesses that none has the right to be worshipped but Allah and that I am His Apostle, cannot be shed except in three cases: In Qisas for murder, a married person who commits illegal sexual intercourse and the one who reverts from Islam (apostate) and leaves the Muslims
Reference: Bukhari 6878