Musannaf Ibn Abi Shayba 3:3553

Muhammad ibn Bishr (Bashir) narrated to us, he said: Sa’id narrated to us, from Qatadah, from Anas, regarding the saying of the Exalted: "Their sides forsake their beds," he said: They used to perform voluntary prayers in the time between the two prayers, Maghrib and Isha, so they would pray.