Committing this act is called "takfir" and it involves declaring oneself to be a kafir.
According to Ibn Ishaq, Ubayd-Allah ibn Jahsh committed this sin after moving to Abyssinia. In one Hadith, Muhammad refused to sit down because Abu Musa brought before him a Jew who had committed this sin. Sharia divides this sin into "fitri" and "milli" varieties and somebody who commits this sin is termed a "murtad". Committing this act is called "takfir" and it involves declaring oneself to be a kafir. This sin is one of three for which a Muslim may be executed, along with murder and extramarital affairs For ten points, name this act of leaving one's religion, most notably practiced by Roman Emperor Julian.
According to Ibn Ishaq, Ubayd-Allah ibn Jahsh committed this sin after moving to Abyssinia.
{apostacy} or {being} an {apostate} [accept: {ridda}]