Is a pilgrim's Hajj valid, and what are the implications, if he stones the Jamarat on the twelfth day before Fajr prayer?

Casting the pebbles, or some of them, or casting them at an incorrect time, does not invalidate the Hajj. Casting before the dawn of the second day of the Days of Tashreeq is not considered valid in Sharia. The time for casting the pebbles on the Days of Tashreeq does not begin until after the sun declines, according to the majority of scholars, and this is the most preponderant opinion. Whoever casts before the sun declines must repeat the casting. Whoever departs early on the twelfth day before casting must offer a damm (sacrifice) for abandoning the casting, but is not required to offer a damm for abandoning the stay on the night of the thirteenth.