Musnad Abu Ya'la al-Mawsili 109:132

Ishaq narrated to us, Muhammad ibn Khazim narrated to us, al-A'mash narrated to us, from Abu Sufyan, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) was asked: "Which prayer is best?" He said: "The length of the Qunut (standing)."