Sunan Ibn Majah, Chapter: The Chapters on the Mosques and the Congregations

"We performed the Maghrib (prayer) with the Messenger of Allah (saw), then those who went back went back, and those who stayed, stayed. Then the Messenger of Allah (saw) came back in a hurry, out of breath, with his garment pulled up to his knees, and said: 'Be of good cheer, for your Lord has opened one of the gates of heaven and is boasting of you before the angels, saying: "Look at My slaves; they have fulfilled one obligatory duty and are awaiting another." (Sahih)