Authentic
Sunan Ibn Majah 871
“We set out until we came to the Messenger of Allah (ﷺ), and we gave him our oath of allegiance and performed prayer behind him. He glanced out of the corner of his eye at a man who was not settling his spine when he bowed and prostrated. When the Prophet (ﷺ) finished the prayer, he said: ‘O…
لاَ صَلاَةَ لِمَنْ لاَ يُقِيمُ صُلْبَهُ فِي الرُّكُوعِ وَالسُّجُودِ "