بُلُوغُ الْمَرَامِ
Bulugh al-Maram 195
The Book of Prayer
وَعَنْ عُثْمَانَ بْنِ أَبِي الْعَاصِ - رضى الله عنه - { أَنَّهُ قَالَ : يَا رَسُولَ اَللَّهِ اِجْعَلْنِي إِمَامَ قَوْمِي . قَالَ : "أَنْتَ إِمَامُهُمْ , وَاقْتَدِ بِأَضْعَفِهِمْ , وَاِتَّخِذْ مُؤَذِّنًا لَا يَأْخُذُ عَلَى أَذَانِهِ أَجْرًا } أَخْرَجَهُ اَلْخَمْسَةُ , وَحَسَّنَهُ اَلتِّرْمِذِيُّ , وَصَحَّحَهُ اَلْحَاكِمُ . [1]
Narrated 'Uthman bin Abul-'Aas (RA): He said, "O Messenger of Allah, appointment as the Imam (leader) of my people (in prayers). He (ﷺ) said: "You are their Imam, but you should follow (observe the strength of) the weakest among them and appoint a Mu'adhdhin who does not charge for the call of Adhan." [Reported by al-Khamsah, at-Tirmidhi graded it Hasan (good) and al-Hakim graded it Sahih (authentic)].
Sunnah.com reference: Book 2, Hadith 57