Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Answering your question, the late prominent Saudi scholar, Sheikh Ibn `Uthaymeen, states:
There is a difference of opinion among the scholars concerning this issue – i.e., beginning the khutbah and prayer on Friday before the sun passes its zenith. Some of them say that it is not permissible to do so until the sun has passed its zenith, and some of them say that it is permissible.
The correct view is that it is permissible one hour or half an hour before the sun passes its zenith, or something close to that, but it is preferable to do it after the sun has passed its zenith, even according to those who say that it is permissible to bring it forward by an hour or so.
If someone prays before the sun passes the zenith, based on the opinion which says that doing so is permissible before the sun passes its zenith, then there is nothing wrong with it, but it should be only a short time before.
Allah Almighty knows best.