Praise be to Allah.
If someone slaughtered his sacrificial animal (hadiy) before the day of Eid, and came to ask us about that, we would have to ask him: Did you do that by way of following the advice of a scholar, or was it out of heedlessness? If he did it by way of following the advice of a scholar, then he does not have to repeat it, because there are some scholars who think that it is permissible to slaughter the sacrificial animal (hadiy) before Eid when doing tamattu‘. So if this man is following those scholars, or he asked one of the scholars who follow that opinion, and he told him that his sacrifice was valid, then we do not instruct him to repeat the sacrifice.
But if he slaughtered the sacrifice before the day of Eid out of carelessness, and that was not on the basis of knowledge or following a scholar, then he must repeat the sacrifice, because it is not permissible to slaughter the sacrifice for tamattu‘ or qiraan except on the day of Eid or the following days. The evidence for that is that if it were permissible to slaughter the sacrifice before the day of Eid, the Prophet (blessings and peace of Allah be upon him) would have slaughtered his sacrifice before then and exited his ihram, as he instructed his companions to do. In fact, he (blessings and peace of Allah be upon him) said: “I have the sacrificial animal with me, so I will not exit ihram until I slaughter it.” If it were permissible to slaughter the sacrificial animal before the day of Eid, he would have slaughtered it then exited ihram. End quote.
Majmoo‘ Fataawa Ibn ‘Uthaymeen (23/163).