Musnad Abu Ya'la al-Mawsili 109:104

Muhammad ibn al-Khattab narrated to us, Mu'ammal narrated to us, Shu'bah narrated to us, Sa'd ibn Ibrahim narrated to us, from Muhammad ibn 'Amr, who said: When al-Hajjaj arrived, he began to delay the prayer. So I asked Jabir—or he was asked—and he said: The Messenger of Allah (peace be upon him) used to pray the Dhuhr at the midday heat, and he would pray the 'Asr while the sun was still bright, and he would pray the Maghrib when the sun had set. As for the 'Isha, if the people had gathered, he would hasten it, and if the people were few, he would delay it. And he would pray the Subh (Fajr) in the darkness before dawn.