Sahih al-Bukhari 389
Sahih al-Bukhari · Book 8: Prayers (Salat) · If someone does not prostrate properly
Narrated Hudhaifa that he saw a person bowing and prostrating imperfectly. When he finished his Salat, Hudhaifa told him that he had not offered Salat. The subnarrator added, "I think that Hudhaifa also said:
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.
Were you to die you would die on a "Sunna" (legal way) other than that of Muhammad (ﷺ)."