Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was asked about the ruling on offering a prayer before the time for it has begun. 

He replied: 

Offering a prayer before the time for it has begun is not valid, even if it was only one minute before the time for it. If a person says the opening takbeer before the time, then his prayer is not valid, because Allaah says (interpretation of the meaning):

 “Verily, As‑Salaah (the prayer) is enjoined on the believers at fixed hours”

[al-Nisa’ 4:103] 

i.e., strictly-defined times. So the prayer is not valid if it is offered before its time, and the prayer that you offered before its time must be repeated. And Allaah is the source of strength. End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen (12/216). 

He also said: Prayer is not valid if done before its time, according to the consensus of the Muslims. If a person offers a prayer ahead of time, if he did that deliberately then his prayer is invalid, and he is not free of sin. If he did not do it deliberately, because he thought that the time for it had begun, then he is not a sinner, and his prayer is regarded as naafil (supererogatory), but he has to repeat it, because one of the conditions of prayer is that the time for it should have begun. End quote from al-Sharh al-Mumti’.