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].

وَعَنْ جَابِرٍ ‏- رضى الله عنه ‏- قَالَ: { مَضَتِ السُّنَّةُ أَنَّ فِي كُلِّ أَرْبَعِينَ فَصَاعِدًا جُمُعَةً } رَوَاهُ اَلدَّارَقُطْنِيُّ بِإِسْنَادٍ ضَعِيف ٍ [1]‏ .‏