Sa'id ibn Ziyad ibn Subayh al-Hanafi said: I prayed by the side of Ibn Umar and I put my hands on my waist. When he finished his prayer, He said: This is a cross in prayer; the Messenger of Allah ﷺ used to forbid it.  

هَذَا الصَّلْبُ فِي الصَّلاَةِ وَكَانَ رَسُولُ اللَّهِ ﷺ يَنْهَى عَنْهُ