Mu'awiyah bin Salih (may Allah have mercy on him) said that Umar bin Abd al-Aziz (may Allah have mercy on him) wrote: In any village where fifty men gather, one of them should lead them in prayer, deliver the sermon to them, and lead them in the Friday prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5611