The devil circulates in the son of Adam like blood. I feared lest he should cast evil thoughts in your hearts

The Prophet (may Allah's peace and blessings be upon him) was observing I‘tikāf during the last ten nights of Ramadan. His wife, Safiyyah (may Allah be pleased with her) visited him one of those nights and talked to him for some time. Then she got up to go back home, so the Prophet got up with her to escort her and keep her company.

Two men from the Ansār walked by, and when they saw the Prophet (may Allah's peace and blessings be upon him) they sped up out of shyness when they saw his wife with him. So the Prophet (may Allah's peace and blessings be upon him) said to them: Slow down for this is only my wife Safiyyah. The two men said in embarrassment: Subhān Allah! Is it even conceivable that someone might have suspicions about you (i.e. suspecting that the woman with him was not his wife)?

In reply, the Prophet (may Allah's peace and blessings be upon him) informed them that the devil is keen on luring the sons of Adam, and that he is highly capable of doing so, for he runs through their veins just like blood given the subtle ways he sneaks into their hearts and minds. Therefore, he feared that the devil might cast suspicions in their hearts.