Ali bin Abi Taalib RA appointed him as successor (in his absence) over Al-Madinah, then over Iraq. After that he was constantly with him and fought the Al-Khawaarij with him. He said, Allah, the Almighty, said (in Surah 9, p/o verse 41), March forth, whether you are light (being healthy, young and wealthy) or heavy (being ill, old and poor). I am not heavy or light, because of that the struggle becomes necessary. He reported about the Prophet (Peace be upon him) and about the community of Sahaabah and Taabi'yeen.
He died a martyr in Jihaad (struggling in the cause of Allah), in the year 52 AH, in all probability. He was in the army of Yazeed bin Mu'aawiyah bin Abi Sufyaan that battled against Rome. Yazeed went to visit him and said, what do you need, Oh Companion of the Messenger of Allah (Peace be upon him)! He said, (Provide) my needs if I die. I ride and arrive in the earth of the enemy and find what I meet. If you don't find (it), bury me and then return. He died (Allah be pleased with him). Yazeed carried out his wishes as he had been requested, even burying him under the wall of Al-Qastantiniyah. His grave is known. (They [the people there] pray for rain at it. They say it's the grave of a pious man).