Musnad Abu Ya'la al-Mawsili 155:303

Suwayd ibn Sa'id narrated to us, al-Hasan ibn al-Sakan al-Basri narrated to us, from al-A'mash, from Abu Zabyan, from Abu Hurayrah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "For everything there is a quintessence, and the quintessence of the prayer is the first takbir."