It is not permissible to stone before Midday
Question :
Verily, during the last days of Hajj I stoned the pillars 15 minutes before the Athan of Az-Zuhr. Is this the time of midday and is there anything due on me if it (the time of midday) did not start until after this?
Answer:
You must slaughter an animal in Makkah for the poor, because stoning the pillars during the days of At-Tashreeq is only after midday and it does not suffice to do it before that. This is because the Prophet ﷺ stoned during the days of At- Tashreq after midday and he said:
"Take your Hajj rites from me."
Thus, it is obligatory on the Muslims to follow him in that - may the best of blessings and peace be upon him from His Lord.
Along with that, you must repent to Allâh, because you opposed that which was legislated. May Allâh pardon you and us and every Muslim.
Source:
Ash-Shaykh Ibn Baz
Fatawa Islamiyah, Vol. 4 Pages 245-246