Sending of Mu'adh ibn Jabal to Yemen

The Prophet sent Mu'adh ibn Jabal, one of the most learned companions in matters of halal and haram, as a judge, teacher, and governor to Yemen. Before his departure, the Prophet asked him: 'How will you judge when a case comes to you?' Mu'adh replied: 'I will judge by the Book of Allah.' The Prophet asked: 'And if you do not find the answer in it?' He said: 'Then by the Sunnah of the Messenger of Allah.' The Prophet asked: 'And if you do not find it in that?' Mu'adh said: 'I will exercise my own judgment (ijtihad) and spare no effort.' The Prophet struck Mu'adh's chest and said: 'Praise be to Allah who has guided the messenger of the Messenger of Allah to that which pleases the Messenger of Allah.' The Prophet walked alongside Mu'adh's mount as he departed, giving him final instructions: 'You are going to a people of the Book. Let the first thing you call them to be the worship of Allah alone.' In an emotional farewell, the Prophet told him: 'O Mu'adh, perhaps you will not meet me after this year. Perhaps you will pass by this mosque of mine and my grave.' Mu'adh wept profusely.