Musannaf Ibn Abi Shayba 2:55

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from ‘Imran ibn Muslim, from Suwayd ibn Ghafala, that he sent word to a mu’adhdhin of his who was called Rabah, that he should not perform the *tathwib* (the saying of "as-salatu khayrun min an-nawm") except in the dawn (Fajr) prayer.