بُلُوغُ الْمَرَامِ
Bulugh al-Maram 467
The Book of Prayer
وَعَنْ جَابِرٍ - رضى الله عنه - قَالَ: { مَضَتِ السُّنَّةُ أَنَّ فِي كُلِّ أَرْبَعِينَ فَصَاعِدًا جُمُعَةً } رَوَاهُ اَلدَّارَقُطْنِيُّ بِإِسْنَادٍ ضَعِيف ٍ [1] .
Narrated Jabir (RA): It is decided in the Sunnah that the Friday prayer (established/recognized) when forty praying persons or more attend it. [ad-Daraqutni reported it through a weak chain of narrators].
Sunnah.com reference: Book 2, Hadith 389