Musannaf Ibn Abi Shayba 3:5141

Shababa ibn Sawwar narrated to us, from Layth ibn Sa'd, from Ibrahim ibn Abdullah ibn Ma'bad, from Ibn Abbas, from Maymunah: She heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "A prayer in it"—meaning the Mosque of Medina—"is better than a thousand prayers in what is other than it, except for the Mosque of Mecca."