Musannaf Ibn Abi Shayba 2:53

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Sufyan, from Ibn al-Asbahani, from Abd al-Rahman ibn Abi Layla, who said: "They did not innovate an innovation more beloved to me than the *tathwib* (calling out) in the prayer," meaning the Isha and the Fajr.