Musannaf Ibn Abi Shayba 3:6

Abu Bakr narrated to us, he said: Ibn Fudayl and Waki’ narrated to us, from Mis’ar, from Uthman al-Thaqafi, from Salim, who said: Abu al-Darda’ said: "For everything there is a slogan (shi’ar), and the slogan of prayer is the takbir."