Narrated Wabrah:

I asked Ibn 'Umar: When should I throw pebbles at the jamrah? He replied: When your imam (leader at Hajj) throws pebbles, at that time you should throw them. I repeated the question to him. Thereupon he said: We used to wait for the time when the sun passes the meridian. When the sun declined, we threw the pebbles.

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُحَمَّدٍ الزُّهْرِيُّ، حَدَّثَنَا سُفْيَانُ، عَنْ مِسْعَرٍ، عَنْ وَبَرَةَ، قَالَ سَأَلْتُ ابْنَ عُمَرَ مَتَى أَرْمِي الْجِمَارَ قَالَ إِذَا رَمَى إِمَامُكَ فَارْمِ ‏.‏ فَأَعَدْتُ عَلَيْهِ الْمَسْأَلَةَ فَقَالَ كُنَّا نَتَحَيَّنُ زَوَالَ الشَّمْسِ فَإِذَا زَالَتِ الشَّمْسُ رَمَيْنَا ‏.‏