Praise be to Allah.
The Standing Committee for Issuing Fatwas was asked the following question:
What is the number of men required for Jumuah prayer to be valid? Some people say that it is not valid with less than forty men, and if there is one less than that, they should pray Zuhr.
Answer: Establishing Jumuah prayers is obligatory (waajib) upon the Muslims in their villages on Fridays, and it is necessary for there to be a jamaaah (congregation) in order for the Jumuah prayer to be valid. There is no proven evidence (daleel) which states that a specific number is required; for the prayer to be valid it is sufficient for there to be three or more people. It is not permissible for a person who is obliged to pray Jumuah to pray Zuhr in its place when there are less than forty people present according to the correct scholarly view. And Allaah is the source of strength. May Allaah bless our Prophet Muhammad and his family and companions, and grant them peace.