Sahih Bukhari Volume 1, Book 8, Hadith 389

أَخْبَرَنَا الصَّلْتُ بْنُ مُحَمَّدٍ، أَخْبَرَنَا مَهْدِيٌّ، عَنْ وَاصِلٍ، عَنْ أَبِي وَائِلٍ، عَنْ حُذَيْفَةَ، رَأَى رَجُلاً لاَ يُتِمُّ رُكُوعَهُ وَلاَ سُجُودَهُ، فَلَمَّا قَضَى صَلاَتَهُ قَالَ لَهُ حُذَيْفَةُ مَا صَلَّيْتَ ـ قَالَ وَأَحْسِبُهُ قَالَ ـ لَوْ مُتَّ مُتَّ عَلَى غَيْرِ سُنَّةِ مُحَمَّدٍ صلى الله عليه وسلم

Narrated Hudhaifa that he saw a person bowing and prostrating imperfectly.When he finished his Salat(prayer), Hudhaifa told him that he had not offered Salat.The subnarrator added,"I think that Hudhaifa also said : Were you to die you would die on a "Sunna" (legal way) other than that of Muhammad(s.a.w.)."