Sahih al-Bukhari 1746
Narrated Wabra:
I asked Ibn `Umar, "When should I do the Rami of the Jimar?" He replied, "When your leader does 
that." I asked him again the same question. He replied, "We used to wait till the sun declined and then 
we would do the Rami (i.e. on the 11th and 12th of Dhul-Hijja)."
حَدَّثَنَا أَبُو نُعَيْمٍ، حَدَّثَنَا مِسْعَرٌ، عَنْ وَبَرَةَ، قَالَ سَأَلْتُ ابْنَ عُمَرَ ـ رضى الله عنهما ـ مَتَى أَرْمِي الْجِمَارَ قَالَ إِذَا رَمَى إِمَامُكَ فَارْمِهْ‏.‏ فَأَعَدْتُ عَلَيْهِ الْمَسْأَلَةَ، قَالَ كُنَّا نَتَحَيَّنُ، فَإِذَا زَالَتِ الشَّمْسُ رَمَيْنَا‏.‏