Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 47
Question: If a person is ignorant about the time of the prayer and prays before its appointed time, then what is the ruling [of the prayer he has just performed]?
Response: A person’s [performance of the] prayer before it’s appointed time does not relinquish him of it’s obligation as Allaah (Subhaanahu wa Ta’aala) says:
And the Prophet (sal-Allaahu ‘alayhi wa sallam) has explained these times (fixed hours) in his saying:
So, based upon this, whoever performs the prayer before its appointed time then his [performance of the] prayer does not relinquish him from its obligation. However, [the performance of this obligatory prayer before its appointed time] is regarded as a voluntary prayer, meaning that it is rewarded as a voluntary prayer and he is required to repeat the [obligatory} prayer at it’s [appointed] time, and Allaah [alone] has the Complete knowledge [of all affairs].