Musannaf Ibn Abi Shayba 2:40

Abu Bakr narrated to us, he said: Hafs bin Ghiyath narrated to us, from Hajjaj, from Ata, from Abu Mahdhura. And from Ata, from Suwayd, from Bilal, that the end of their *tathwib* (call to prayer) was: "Prayer is better than sleep."