Musnad Abu Ya'la al-Mawsili 111:66

Musa narrated to us, Yahya bin Sa'id narrated to us, from Dawud bin Qays, Salih, the freed slave of al-Taw'amah, narrated to me, 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, without being in a state of travel or rain. He (the narrator) said: I said, "What did he intend by that?" He said, "He intended to provide ease for his nation."