It was narrated that Ibn 'Abbas (رضي الله عنه) said:

The Messenger of Allah (ﷺ) put Zuhr and ‘Asr together, and Maghrib and ‘Isha', in Madinah when there was no fear and no rain. I (the narrator) said to Ibn 'Abbas: Why did he do that? He said: So as not to make things difficult for his ummah.