Musnad Abu Ya'la al-Mawsili 160:51

Abu Hammam narrated to us, Kulthum bin Muhammad bin Abi Sidrah narrated to us, that Ata' al-Khurasani narrated to them from Abu Hurairah, that the Messenger of Allah (may Allah bless him and grant him peace) used to say: "Part of the excellence of the prayer is the length of the Qunut."