It was narrated from Ibn`Abbas that the Messenger of Allah said:
"Whoever denies a Verse of the Qur'an, it is permissible to strike his neck (i.e., execute him) Whoever says, Lailaha illallahu wahduhu la sharika lahu, wa anna Muhammadan `abduhu wa rasuluhu (None has the right to be worshiped but Allah (SWT) alone, and Muhammad (ﷺ) is His slave and Messenger), no one has any was of harming him, unless he (does something which) deserves a legal punishment, and it is carried out on him."
English reference: Vol. 3, Book 20, Hadith 2539
Arabic reference: Book 20, Hadith 2636