Hadith 142

Bulugh al-Maram

Book 2: The Book of Prayer
وَفِي لَفْظٍ لِأَحْمَدَ : { فَأَقِمْ صُلْبَكَ حَتَّى تَرْجِعَ اَلْعِظَامُ } 1‏
And Ahmad has:

"Keep your back straight till the bones return (to their positions)."