Yahya related to me from Malik from Ibn Shihab from Ali ibnHusayn ibn Ali ibn Abi Talib that the Messenger of Allah, may Allahbless him and grant him peace, said, "Part of the excellence of aman's Islam is that he leaves what does not concern him."
USC-MSA web (English) reference: Book 47, Hadith 3
Arabic reference: Book 47, Hadith 1638