The Messenger of Allah ﷺ said, "If a slave flees to the land of polytheism, his blood becomes lawful."
أخبرنا قتيبة قال حدثنا حميد بن عبد الرحمن عن أبيه عن أبي إسحق عن الشعبي
قال رسول الله ﷺ إذا أبق العبد إلى أرض الشرك فقد حل دمه
[Sahih] Narrated by Abu Dawud, Hudud, chapter: The ruling on one who apostatizes, hadith no. 4360, from Qutayba, with this wording. It is also found in al-Kubra, hadith no. 3515, and this hadith has corroborating narrations.