Abdullah b. Abbas reported that he saw 'Abdullah b. al-Harith observing the prayer and (his hair) was plaited behind his head. He ('Abdullah b. 'Abbas) stood up and unfolded them. While going back (from the prayer) he met Ibn 'Abbas and said to him: Why is it that you touched my head? He (Ibn 'Abbas) replied: (The man who observes prayer with plaited hair) is like one who prays with his hands tied behind.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَمْرُو بْنُ سَوَّادٍ الْعَامِرِيُّ، أَخْبَرَنَا عَبْدُ اللَّهِ بْنُ وَهْبٍ، أَخْبَرَنَا عَمْرُو بْنُ الْحَارِثِ، أَنَّ بُكَيْرًا، حَدَّثَهُ أَنَّ كُرَيْبًا مَوْلَى ابْنِ عَبَّاسٍ حَدَّثَهُ عَنْ عَبْدِ اللَّهِ بْنِ عَبَّاسٍ، أَنَّهُ رَأَى عَبْدَ اللَّهِ بْنَ الْحَارِثِ يُصَلِّي وَرَأْسُهُ مَعْقُوصٌ مِنْ وَرَائِهِ فَقَامَ فَجَعَلَ يَحُلُّهُ فَلَمَّا انْصَرَفَ أَقْبَلَ إِلَى ابْنِ عَبَّاسٍ فَقَالَ مَا لَكَ وَرَأْسِي فَقَالَ إِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ " إِنَّمَا مَثَلُ هَذَا مَثَلُ الَّذِي يُصَلِّي وَهُوَ مَكْتُوفٌ " .
sunnah.com