Abdullaah bin Mubaarak - may Allah have mercy upon him - said:
I sat out for hajj to the sacred house of Allah and visiting the masjid of His Prophet may Allah's peace and blessings be upon him. As I was in the road, I suddenly saw a black figure and when I managed to distinguish it I...
This has to do with the early muslim community in Makkah, they raised their voices in fajr becuase the mushrikeen where asleep and could not hear them but duhur and asr there was the possibilty of being over heared and face torment, as for magrib then the people where busy with theyre animals...