Atiyyah bin Qais (may Allah have mercy on him) said: "If any companion joins the enemy and commits murder, adultery, or theft, then seeks amnesty, the matter from the time of polytheism is forgiven. But if he does so in Islam, then turns to polytheism, the prescribed punishment will be carried out upon him."
Hadith Referenceسنن سعید بن منصور / كتاب الجهاد / 3981
Hadith Gradingمحدثین:إسناده ضعيف
Hadith Takhrij«إسناده ضعيف، و«انفرد به المصنف من هذا الطريق»» قال ابن حجر: أبي بكر بن أبي مريم ضعيف
Brief Explanation
This narration is weak and mursal, therefore it is not an independent proof. However, its meaning is supported by other authentic texts, so it is worth mentioning for the purpose of corroboration (istinaas).