Narrated from Sayyiduna Naufal Ashja'i (may Allah be pleased with him), he says: The Noble Prophet (peace and blessings be upon him) gave me the daughter of Umm Salamah (may Allah be pleased with her) and said: You are the husband of the woman who nursed my daughter. She remained with me for as long as Allah willed, then when I came to the Prophet (peace and blessings be upon him), he asked: What happened to the girl? I said: She is with her mother. The Prophet (peace and blessings be upon him) said: Why have you come now? I said: Teach me what I should say when I go to sleep. The Prophet (peace and blessings be upon him) said: Recite Surah Al-Kafirun when you go to sleep, and when you finish it, then go to sleep. This is a declaration of disassociation and freedom from shirk (polytheism).