Nahjul Balagha Letter 44

To Ziad ibn Abih when Ameerul Momineen had come to know that Mnawiya had written to Ziad to deceive him and to attach him to himself in kinship.

I have learnt that Muawiya has written to you to deceive your wit and blunt your sharpness. You should be on guard against him because he is the Satan who approaches a believer from the front and from the back, from the right and from the left, to catch him suddenly in the hour of his carelessness and overcome his intelligence. In the days of Umar bin Khattab, Abu Sufyan happened to utter a thoughtless point which was an evil suggestion of Satan, from which neither kinship is established nor entitlement to succession occurs. He who relies on it is like the uninvited guest to a drink-party or like the dangling cup (tied to a saddle).