Musnad Ahmad 1034:119

'Affan narrated to us, Mahdi narrated to us, Wasil al-Ahdab narrated to us, from Abu Wa'il, from Hudhayfah, that he saw a man who was not completing his bowing (ruku') nor his prostration (sujud). When he finished his prayer, Hudhayfah called him and said to him: "How long have you been praying this prayer?" He said: "I have prayed it for such and such [a time]." Hudhayfah said: "You have not prayed," or he said: "You have not prayed a prayer for Allah." Mahdi was uncertain, and I think he said: "And if you were to die, you would die upon other than the Sunnah of Muhammad, may Allah bless him and grant him peace."