´It was narrated from 'Abdullah bin 'Abbas that:` He saw 'Abdullah bin Al-Harith praying with his hair bound behind him. He stopped and started to undo it. When he finished he turned to Ibn 'Abbas and said: "What were you doing to my head?" He said: "I heard the Messenger of Allah (ﷺ) say: 'The likeness of this is that of one who prays with his hands tie behind his neck.'"
Hadith Referenceسنن نسائي / كتاب التطبيق / 1115
Hadith Gradingالألبانی:صحيح | زبیر علی زئی:صحيح مسلم
➊ Just as a person whose hands are tied behind his back performs a very deficient prayer, similarly, one whose hair is tied up deprives his hair of reward; whereas, if the hair were touching the ground, their prostration (sajdah) would also be counted and they would receive reward. It is as if one should not have their hair tied up even before the prayer, let alone doing so during the prayer.
➋ Upon witnessing an act contrary to the Shari‘ah, one should immediately offer a warning on the spot; one should not remain silent without reason or altogether.
➌ If one has the ability to remove an evil with his hand, then he should remove it with his hand.
➍ A solitary report (khabar wahid) is a valid proof.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 1115
Narrated Abdullah ibn Abbas: Kurayb the freed slave of Ibn Abbas reported: Abdullah ibn Abbas saw Abdullah ibn al-Harith praying having the back knot of the hair. He stood behind him and began to untie it. He remained standing unmoved (stationary). When he finished his prayer he came to Ibn Abbas and said to him: What were you doing with my head? He said: I heard the Messenger of Allah ﷺ say: A man who prays with the black knot of hair tied is the one praying pinioned.
Shaykh Umar Farooq Saeedi
647. Commentary: ➊ For men, tying the hair into a knot, especially during prayer, is not permissible; rather, they should leave it loose as it is and allow it to touch the ground in the state of prostration. In another hadith, it is explicitly stated: "I have been commanded to prostrate on seven bones and not to tie the hair or gather up the garment." [صحيح بخاري، حديث : 812 و صحيح مسلم، حديث : 490] ➋ As for those elders about whom it is reported that they had tied their hair, perhaps they were unaware of this Prophetic instruction.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 647