Narrated Abu Hurairah (radiAllahu anhu): "Allah’s Messenger (salAllahu alayhi wasalam) talked about Friday and said •► ‘There is an hour on Friday and if a Muslim gets it while offering Salat (prayer) and asks something from Allah (Subhanahu wa Ta’ala), then Allah (Subhanahu wa Ta’ala) will definitely meet his demand.’
And he (the Prophet (salAllahu alayhi wasalam) pointed out the shortness of that particular time with his hands." [Bukhari :: Book 2 :: Volume 13 :: Hadith 57]
Some have said that this hour is from the time the Imam (prayer’s leader) enters the mosque on Friday’s prayer until the prayer is over (ie between the two khutbahs), whereas others have said that it is the last hour of the day (i.e. after the Asr prayer until the Maghrib prayer).
(Note: when we say an hour here we do not mean the hour everyone knows (60 minutes) but an unspecified period of time because the time counters which we use today came after the Prophet (salAllahu alayhi wasalam) had died.