Mishkat al-Masabih 4 Prayer كتاب الصلاة 423

وَعَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «الِاخْتِصَارُ فِي الصَّلَاةِ رَاحَةُ أَهْلِ النَّارِ» . رَوَاهُ فِي شرح السّنة

Ibn ‘Umar reported God’s Messenger as saying, “Resting the hands on the waist in prayer is the comfort of those who go to hell.” [Baghawi] transmitted it in Sharh as-sunna .