Assalamu alaikum.Is it not allowed for women to tie a knot on her hair at the back of her head while making Wudhu or praying?even if she is wearing hijab?Is there any evidence in Fiqh?Salaam.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

We are not aware of any evidence which prohibits a woman from praying while having her hair knotted in the back. The narration which prohibits praying while having one's hair knotted is in regard to men but not women. Raafi' narrated: "The Prophet prohibited that a man pray while his hair is knotted." [Ahmad]  

Moreover, Imaam Muslim reported that Ibn 'Abbaas saw 'Abdullaah Ibn Al-Haarith praying while his hair is knotted in the back, so he went to him and untied his knot. When the man finished his prayer he told Ibn 'Abbaas: "What was wrong with my hair?" Thereupon, Ibn 'Abbaas said: "I heard the Prophet saying: "The person who performs the prayer in this condition is like the one who performs the prayer while being handcuffed."  

For more benefit, please refer to Fatwa 90963.

Besides, Al-Kaasaani from the Hanafi School of jurisprudence, said: ''To knot one's hair is to tie it like a braid around his head exactly as women do, or to tie a hair and make a knot with it in the back of his head."

Furthermore, Al-Haafith Al-'Iraaqi said: "The above narration addresses men and not women, because the hair of a woman is a part of 'Awrah (parts of the body that must be covered in Islam) which must be covered during the prayer because if she unties it, it might appear and will be difficult for her to cover it, which invalidates her prayer. In addition to this, it is difficult for a woman to untie her hair every time she stands to pray." The Prophet even permitted the women not to untie their braids when performing Ghusl (i.e. ritual bath), despite the need of wetting all the hair." For more benefit, please refer to Fatwa 89307.

Therefore, it becomes clear that the probation of having one's hair knotted while praying is in regard to men and does not apply to women.

Finally, it should be noted that if a woman knots her hair in a way which resembles dissolute women or non-Muslim women, or does it is like a camel's hump, then in this case, she is prohibited from knotting her hair in such a manner whether it is during the prayer or outside the prayer as it is prohibited to imitate dissolute or non-Muslim women.

Allaah Knows best.