Q.316. When does the first hour of the Friday prayer begin?
A. The hours which the Messenger (صلّى الله عليه و سلّم) has mentioned are five in number; he said:
“Whoever bathed on Friday, performing the Ghusl of major ritual impurity (Janabah), then went (to the Masjid) in the first hour, he is like one who offers a camel as a sacrifice, and he who comes at the second hour is like one who offers a cow, and he who comes at the third hour is like one who offers a ram with horns, and he who comes at the fourth hour is like one who offers a hen, and he who comes at the fifth hour is like one who offers an egg.” 
So, he has divided the time from sunrise until the arrival of the Imam into five parts. Each part might be equivalent to the well known hour (i.e. 60 minutes), or the hour might be more or less, because the time changes, so the hours are five between the sunrise and the arrival of the Imam for the prayer. It begins from sunrise, although it has been said that it is from the start of Fajr, but the former is more likely, because before sunrise is the time for the Fajr prayer.
 Reported by Al-Bukhari in the Book of the Friday Prayer, in the Chapter: The Virtue of the Friday Prayer (881); and by Muslim in the Book of the Friday Prayer, in the Chapter: Perfume and Siwak on Friday (850).
Source: Fatwa Arkanul Islam, Volume 2 – Shaykh Uthaymeen, Dar-us-Salam English Publication