In The Name of Allaah, The Most Merciful, The Bestower of Mercy
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.’’ [Ref 1] 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.
[An Excerpt from ‘Sharh Hilyati Taalibil Ilm’ page 346. Slightly paraphrased]
Ref 1: Al-Bukhaari 5146