"The Messenger of Allah ﷺ said: 'If a man feels drowsy when he is praying, let him stop, lest he supplicate against himself without realizing.'" (Using translation from Nasāʾī 162)
"إذَا نَعَسَ الرَّجلُ وهُوَ يُصلِّى فلينصرفْ، فلعَّله يَدْعو علَى نفْسه وهو لا يدْرِى".