Sufyan ibn Husayn al-Wasiti says: I spoke ill of a person in the presence of Iyas ibn Mu’awiyah al-Muzani, who was the judge of Basra, a Tabi’i and a very intelligent person. So he looked at me in the face and asked: ‘Did you wage war against the Romans?’ I replied: ‘No.’ He asked: ‘Did you wage war against Sind, India and the Turks?’ I replied: ‘No.’ He said: ‘How is it that the Romans, the Sindis, the Indians and the Turks are safe from you while your own Muslim brother is not safe from you?’ Sufyan says: ‘ I never did this again, i.e. I never found fault with anyone nor did I speak ill of anyone.'” 
 : Ibn Kathir, al-Bidayah wan-Nihayah.
(The Sunnah way of the Sufis, Imam Abu Harith al-Muhasibi, researched and explained by Shaykh ‘Abd al-Fattah Abu Ghudda)