Musannaf Ibn Abi Shayba 3:5853

Waki’ narrated to us, he said: Dawud ibn Qays al-Farra’ narrated to us, from Salih, the freed slave of al-Taw’amah, from Ibn Abbas, who said: The Messenger of Allah (may Allah bless him and grant him peace) combined the Dhuhr and ‘Asr prayers, and the Maghrib and ‘Isha’ prayers, in Medina, without there being any fear or rain. He said: So it was said to Ibn Abbas, "Why did he do that?" He said: "He wanted to provide ease for his nation."