Mishkat al-Masabih
Abu Huraira said that God’s Messenger forbade putting the hands on the waist during prayer. (Bukhari and Muslim.)
وَعَنْ أَبِي هُرَيْرَةَ قَالَ: نَهَى رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَن الخصر فِي الصَّلَاة "
Reference