The basic ruling is that the mosque that starts the Jumuah prayer first is the valid one. What is the definition of earliest Friday prayer?
When the khatib comes to the musalla?
the adhan is called?
he stands on the minbar?
the actual prayer?
Our teacher Dr. Wahbat al-Zuhayli (may Allah grant him Paradise) wrote in al-Fiqh al-Islami wa-Adillatuh (4th ed. 2:1301):
والسبق يكون بتكبيرة الإحرام، كما قال الشافعية، لا بالشروع في الخطبة ولا بالسلام
“Precedence is defined according to the takbirat al-ihram as the Shafiis said, not by the start of khutba and not by the [final] salam.”
Takbirat al-ihram means the formal start of the two rak`ats of the Jumua prayer.
Hajj Gibril Haddad