Musannaf Ibn Abi Shayba 2:47

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Isma’il bin Abi Khalid, from al-Qasim bin Abi Mikhmarah, that he used to say in his Adhan, during the tathwib: "Prayer is better than sleep, prayer is better than sleep."