Wabra reported,

I asked Ibn Umar, "When should I throw the pebbles at the Jamarat?"He replied, "Throw them when your imam throws."I repeated the question to him, and he said, "We used to watch for the right time, and when the sun passed its zenith, we would throw."

عن وبرة قال

سألت ابن عمر رضي الله عنهما: متى أرمي الجمار؟ قال: إذا رمى إمامك فارمه، فأعدت عليه المسألة، قال: كنا نتحين، فإذا زالت الشمس رمينا.