Sayyiduna Muslim bin al-Ala' bin al-Hadrami (may Allah be pleased with him) said: I was present when the Messenger of Allah (peace and blessings be upon him) gave instructions to al-Ala' bin al-Hadrami as he was sending him to Bahrain. He (peace and blessings be upon him) said: "It is not permissible for anyone to remain ignorant of the obligatory duties and the Sunnahs, and anything besides that is permissible for him." And he (peace and blessings be upon him) wrote to al-Ala': "Treat the Magians (Majus) as the People of the Book are treated."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 17108