Taken from [The Sunnah Way of the Sufis -Imam Harith al-Muhasibi (rahimahullah) researched and explained by Shaykh Abdul Fattah Abu Ghuddah (rahimahullah) translated by Mahomed Mahomedy (Zam Zam publishers)]
Ibn Aun al-Basri kills a roman while concealing himself
Abdullah ibn Aun al-Basri rahimahullah was a great imam; leader, jurist, scholar and memorizer of hadith, scholar of basra, an ardent worshipper, ascetic person and mujahid. He was one of the luminaries of Basra in his memory, jurisprudence, worship and virtues. He was born in 66 A.H. and passed away in 151 A.H.
“Mufaddal ibn Lahiq said: ‘We were in the Roman lands when a Roman came forward challenging (us) to fight him. A man went foward, accepting this challenge, and killed him. This man then went away among the people in order to conceal himself, so that he may not be recognized. I went behind him in order to establish his identity. He had a helmet on his head. He then removed his helmet in order to wipe his face. I saw that it was ibn Aun.’”
[adh-Dhahabi: Siyar A’lam an-Nubala vol6, p368]