Adopting a madh-hab is permissible by ijmaa’
Imam Najm ad-Din Al-Tufi Al-Hanbali [d.716 AH]- may Allah have mercy on him – was a Hanbali scholar who was also a student of ibn Taymiyyah. He said in: “Sharh Mukhtasar Al-Rawda” (3/686): “Giving preference [tarjeeh] to a school of thought [madh-hab] – in its entirety over another madh-hab – has occurred by consensus [ijmaa’], and that is the definite proof of its permissibility, because the four schools of thought and other madh-habs such as Sufyan and Dawood are distributed amongst the Muslims. Such that those who think well of a madh-hab worship Allah based on it and adopt it as Deen, such that the madh-hab of Imam Malik became predominant among the people of the West, the madh-hab of Imam Abu Hanifa over the people of the East, the Shafi’i madh-hab among the majority of the countries between them, and the madh-hab of Imam Ahmad over the people of Gilan [Iraq], so anyone who adheres to a particular madh-hab, that is due to it being most correct according to him either through ijtihad (independent judgment) or taqleed (following a mujtahid), and Muslims unanimously agree upon not rebuking anyone who strictly adheres to any madh-hab he wishes based on that preference.”
So the next time an ignorant person claims that adopting a madh-hab is bid’ah tell him you are the innovator [mubtadi’] for going against the agreement of the scholars [ijmaa’].
Source of page reference: https://ia800203.us.archive.org/23/items/waq58115_701/03_58117.pdf