There is a difference of opinion among the scholars about leaving the hair during Salat.  A group of Ulama called it mustahab or good.

they might have written it because of the following hadith:

  قال رسول الله صلعم : أُمِرْتُ أَنْ أَسْجُدَ عَلَى سَبْعَةِ أَعْظُمٍ, وَلَا أَكُفَّ ثَوْبًا وَلَا شَعْرًا».

 ” Rasulollah (pbh) said: I was ordered to prostrate on seven limbs and not to tie my clothes or hair in prayer.” – Sahih Bukhari: 816, Sahih Muslim: 490

The prohibition stated in this hadith applies to men.  Women are not included in this hadith.

If one braids his hair before praying or has already done so, it is not obligatory for him to leave the hair.

 However, it is not obligatory that because of this the prayer will be makruh or the prayer will be invalidated or the virtue of the prayer will be reduced.  Such matters are not mandatory.

If one ties or looses his hair before prayer, in both cases his hair should be covered.  If you tie your hair first, then the prayer will be purified, and if you leave your hair and cover it, then the prayer will also be purified.