Musnad Ahmad 25:116

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Habib, from Sa'id ibn Jubayr, from Ibn Abbas, who said: "The Messenger of Allah, may Allah bless him and grant him peace, combined the Dhuhr and the 'Asr, and the Maghrib and the 'Isha' in Medina, without being in a state of fear or rain." It was said to Ibn Abbas: "And what did he intend by that?" He said: "He intended that his nation should not be put to hardship."