Wa `alaykum As-Salamu waRahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- The prayers you performed after using a dry hair brush made of boar bristles would not have to be repeated.
3- If one’s hair or body comes into contact with a wet boar bristles brush, then one should wash one’s hair and repeat the prayer.
In his answer to your question, Mufti Muhammad Ibn Adam Al-Kawthari, Director and researcher at the Institute of Islamic Jurisprudence (Darul Iftaa, www.daruliftaa.com), Member of the Al-Qalam Shari`ah Scholars Panel, and Adviser on Islamic Banking, states:
A pig is essentially filthy, hence the position of the Hanafi School and that of many other imams, is that the skin, hair, and all other components of the pig remains filthy even after the tanning process is carried out.
Imam Ibn Abidin (Allah have mercy on him) explains:
“(Al-Haskafi’s statement: “Pigskin cannot be purified”) this is due to the fact that a pig is essentially filthy, in that the essence of a pig with all its body-components is filthy, whether dead or alive. Hence, the filthiness is not because of the blood contained in its body like other non-pure animals. Therefore, it does not accept purification (by tanning or other such methods)….” (Radd al-Muhtar, 1/204)
Imam Al-Haskafi (Allah have mercy on him) states specifically with regard to pig-hair:
“The hair of a dead animal other than a pig…….is pure”
Imam Ibn `Abidin (Allah have mercy on him) explains:
“According to Imam Abu Yusuf, which is the established narration in the Hanafi School, the hair of a pig is considered filthy and impure. This is the position chosen by Imam Al-Kasani in his Al-Bada’i` and Imam Al-Mawsili in his Al-Ikhtiyar.
Thus, if one prayed with pig-hair more than the amount of a dirham, one’s prayer would not be valid…” (Radd al-Muhtar ala al-Durr al-Mukhtar, 1/206)
In light of the above, it will not be permitted to acquire and use a hair brush that is made of boar bristles.
If one is relatively certain of the hair brush being predominantly made of pig-hair, then one must avoid using the brush.
If one offers prayer with pig-hair (or any other part of a pig’s body) attached to one’s self or clothing, then one would have to repeat one’s prayer.
However, using dry hair brushes would not make one’s hair filthy, for the coming into contact with dry pig-hair does not transfer the filth to one’s body.
Thus, the prayers you performed after using a dry hair brush made of boar bristles would not have to be repeated.
However, if one’s hair or body comes into contact with a wet boar bristles brush, then one should wash one’s hair and repeat the prayer.
Almighty Allah knows best.