Musnad Ahmad 25:1376

Yahya narrated to us, from Dawud ibn Qays, who said: Salih, the freed slave of al-Taw'amah, narrated to me from Ibn Abbas, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, combined the Dhuhr and the Asr, and the Maghrib and the Isha, without there being rain or travel." They said: "O Abu Abbas, what did he intend by that?" He said: "To provide ease for his Ummah."