Musannaf Ibn Abi Shayba 3:743

Abu Usama narrated to us, from Abu Farwa Yazid ibn Sinan, he said: Abu Ubayd al-Hajib narrated to us, he said: I heard an old man in the Sacred Mosque saying: Abu al-Darda’ said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, for everything there is a pinnacle (anfah), and the pinnacle of prayer is the first takbir, so preserve it." Abu Ubayd said: So I narrated it to Raja’ ibn Haywah, and he said: Umm al-Darda’ narrated it to me.