All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
What you did, which is throwing the Jimaar (pebbles) before Fajr on the 12th Thul-Hijjah, is not valid. You are obliged to throw the Jimaar after Thuhr and then you may leave, or before Thuhr, according to the opinion of the scholars who are of the view that it is permissible to throw before Thuhr. As regards throwing before Fajr, this is not valid and no scholar has ever permitted it, as far as me know.
Throwing the Jimaar on the 12th Thul-Hijjah is one of the obligations of Hajj, and whoever fails to throw one day or two days, or did not throw at all during the days of Tashreeq (i.e. 11th, 12th, and 13th Thul-Hijjah), is obliged to offer a sacrifice. Since your throwing of the pebbles on the 12th Thul-Hijjah was not valid, then you are obliged to offer a sacrifice. The sacrifice is to slaughter a sheep in the Haram (the sacred boundaries of Makkah) and distribute it on the poor of Makkah, and whoever is not able to do so, he should fast for ten days.
For more benefit on whoever failed to throw the pebbles, please refer to Fatwa 89423.