In The Name of Allaah, The Most Merciful, The Bestower of Mercy.
Imaam Sufyaan Ath-thawri [rahimahullaah] said, “The one who sits with a person of innovation is not safe from one of three [calamities]: Either he becomes a trial for other than himself; or something of [misguidance] enters into his heart, and thus he slips by way of that and Allaah enters him into the hell fire; or he says, ‘By Allaah, I don’t care what they say because I am sure of myself’. But whoever feels safe about his religion [i.e. without Allaah’s guidance, protection etc] in the twinkling of an eye, it will be snatched away from him”. [Ref 1]
Shaikh Uthaymeen [rahimahullaah] said:
It is not permissible to sit with a Mubtadi due to the following reasons: Firstly: We fear his evil, because the Prophet [sallal-laahu-alayhi-wasallam] said: ‘’Indeed, some eloquent speech has the influence of magic.’’ [Al-Bukhaari 5146] He can bewitch the intellect until it agrees with his bidah.
Secondly: It encourages [or promotes] the Mubtadi and thus many people gather around him, or [it is said] that such and such person among the people of nobility, high status and prominence sits with him, so this increases his status and he becomes deluded by his bidah and in himself.
Thirdly: A bad suspicion is held about the one who sits with a person of bidah and this might not become known to him until afterwards; for indeed when the people see you going to a person of bidah, they will blame you even if that is not manifested until afterwards. And because of this, it is befitting for the student of knowledge, rather it is obligated on him to avoid sitting with ahlul bidah. [Ref 2]
[Ref 1: Source: البدع والنهي عنها by Ibn Wadaah. page 54]; [Ref 2: An Excerpt from ‘Sharh Hilyati Taalibil Ilm’ page 346. Slightly paraphrased]