Musannaf Ibn Abi Shayba 3:847

Abu Bakr narrated to us, he said: Ghundar narrated to us, from Shu'bah, from Sa'd ibn Ibrahim, from Muhammad ibn 'Amr ibn al-Husayn, from Jabir ibn 'Abd Allah, he said: The Messenger of Allah (may Allah bless him and grant him peace) used to pray Dhuhr at the midday heat, and 'Asr while the sun was pure, and Maghrib when it had set. As for 'Isha', sometimes he would delay it, and sometimes he would hasten it; if he saw that they had gathered, he would hasten, and if he saw that they were slow, he would delay it. And regarding Subh (Fajr), he said: They used to, and the Prophet (may Allah bless him and grant him peace) used to pray it in the darkness of the end of the night.