Mishkat al-Masabih 4 Prayer كتاب الصلاة 328

وَعَن طلق بن عَليّ الْحَنَفِيّ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لَا يَنْظُرُ اللَّهُ عَزَّ وَجَلَّ إِلَى صَلَاةِ عَبْدٍ لَا يُقِيمُ فِيهَا صُلْبَهُ بَيْنَ ركوعها وسجودها» . رَوَاهُ أَحْمد

Talq b. ‘Ali al-Hanafi reported God’s Messenger as saying, “God does not regard the prayer of a servant who does not straighten his spine between his bowing and his prostration.” Ahmad transmitted it.