Wabarah bin Abd al-Rahman (may Allah have mercy on him) said: I asked Ibn Umar (may Allah be pleased with them both): When should I stone the Jamarat? He said: When your imam stones, then you stone as well. I asked him again, and he said: We used to wait for the time, and when the sun passed its zenith, we would stone.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9664