The story of Abu Mihjan (may Allaah be pleased with him)
Question

                I heard one of the shaykhs speaking about enjoining what is good and forbidding what is evil, and saying that it is obligatory upon every Muslim, even the one who has fallen into sin, to enjoin what is good and forbid what is evil. He said that it is not essential for the person who does this to be of sound character, as is well known from the story of Abu Mihjan. My question is, who was Abu Mihjan and what was his story?.
Praise be to Allah.I congratulate you on your keenness to learn 
    and I ask Allaah to bless us and you with beneficial knowledge and righteous deeds. 
Abu Mihjan was one of the companions of the 
    Prophet  (peace and blessings of Allaah be upon him) (may Allaah be pleased with them). 
This Sahaabi was suffering from an addiction 
    to drinking wine. He was brought and flogged, then brought and flogged again, but he knew that this problem did not relieve him of his duty to 
    strive for the cause of Islam. So he went out with the Muslims to al-Qaadisiyyah as a soldier, seeking martyrdom on the battlefield. In 
    al-Qaadisiyyah he was brought to the commander of the army, Sa’d ibn Abi Waqqaas, for having drunk wine. Sa’d detained him so that the Muslims 
    ranks would be cleansed of such a person.  
This detention was a harsh punishment which 
    caused Abu Mihjan a great deal of anguish. When he heard the sounds of swords and spears, and the neighing of the horses, and he knew that the 
    jihad had begun, and the gates of Paradise were open, he was filled with longing for jihad.  He called to the wife of Sa’d ibn Abi Waaqqas saying, 
    “Let me go and I promise Allaah that if I come back safe and sound, I will put my own feet in the chains, and if I am killed, then you will be rid 
    of me.” She felt sorry for him, so she let him go, and he leapt onto a horse belonging to Sa’d which was called al-Balqa’. Then he picked up a 
    spear and set off. He did not attack any group of enemy soldier but he scattered them. Sa’d was supervising the battle and he was surprised and 
    said, “This is the running of al-Balqa’, and the style of attack is that of Abu Mihjan, but Abu Mihjan is in chains.”  
When the battle was over, Abu Mihjan went 
    back and put his feet in the chains. The wife of Sa’d told him this wonderful story, so Sa’d admired this man and his care for Islam and his 
    longing for jihad, so he himself went to this wine-drinker, released the chains with his own hands and said, “Get up, for by Allaah I will never 
    flog you for drinking wine again.” Abu Mihjan said, “By Allaah, I will never drink it again.” 
See al-Isaabah fi Tamyeez al-Sahaabah, 
    4/173-174; al-Bidaayah wa’l-Nihaayah, 9/632-633