Sahih Bukhari Volume 3, Book 33, Hadith Number 254.
Narated By ‘Ali bin Al-Husain : From Safiya, the Prophet’s wife, The wives of the Prophet were with him in the mosque (while he was in Itikaf) and then they departed and the Prophet said to Safiya bint Huyai, “Don’t hurry up, for I shall accompany you,” (and her dwelling was in the house of Usama). The Prophet went out and in the meantime two Ansari men met him and they looked at the Prophet and passed by. The Prophet said to them, “Come here. She is (my wife) Safiya bint Huyai.” They replied, “Subhan Allah, (How dare we think of evil) O Allah’s Apostle! (we never expect anything bad from you).” The Prophet replied, “Satan circulates in the human being as blood circulates in the body, and I was afraid lest Satan might insert an evil thought in your minds.”