Musnad Ahmad 1163:42

Hajjaj ibn Muhammad narrated to us, he said: Layth ibn Sa’d narrated to us, he said: Nafi’ narrated to me, from Ibrahim ibn Abdullah ibn Ma’bad ibn Abbas, that Maymunah, the wife of the Prophet, may Allah bless him and grant him peace, said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "A prayer in it is better than a thousand prayers in what is other than it of the mosques, except for the mosque of the Kaaba."