The Book of Prayer
Bulugh al-Maram #318
كتاب الصلاة
وَفِي لَفْظٍ لِأَحْمَدَ: «فَأَقِمْ صُلْبَكَ حَتَّى تَرْجِعَ اَلْعِظَامُ»
And Ahmad has:
“Keep your back straight till the bones return (to their positions).”
وَفِي لَفْظٍ لِأَحْمَدَ : "فَأَقِمْ صُلْبَكَ حَتَّى تَرْجِعَ اَلْعِظَامُ"
"Keep your back straight till the bones return (to their positions)."
— Bulugh al-Maram, Hadith 318