Ibn 'Abbas (RAA) narrated that The Messenger of Allah said, “He who changes his religion (i.e. apostates) kill him.” Related by Al-Bukhari.

وَعَنْ اِبْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: قَالَ رَسُولُ اَللَّهِ ‏- صلى الله عليه وسلم ‏-{ مَنْ بَدَّلَ دِينَهُ فَاقْتُلُوهُ } رَوَاهُ اَلْبُخَارِيُّ

English reference : Book 9, Hadith 1242
Arabic reference : Book 9, Hadith 1214