Sayyiduna Uthman bin Abi al-As (may Allah be pleased with him) said: The last advice the Prophet (peace and blessings be upon him) gave me was that he (peace and blessings be upon him) said: "Lead your companions in prayer according to the weakest of them, for among them are the elderly, the weak and those with needs; and appoint a mu'adhdhin who does not take a wage for the call to prayer (adhan)."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 8346