Shaykh Zayd Al-Madkhalee said, “You should not refuse (any innovators request to go to their mosques to give them admonitions and to deliver sermons if you are a Salafi student of knowledge).
If the people of innovations request in their masjid that you deliver a khutbah for them, or to give them an admonition or to establish a conference or a lecture, however it is upon you to target the innovations which they are committing, you should target it (i.e. warn from it) and should you clarify its harm by mentioning the proofs from the verses of the Quran and the hadeeths of the prophet (against it), whereas perhaps Allah may guide by the lecture and the admonition and the khutbah who he wills from his servants to be guided, whereas you get the reward and Allah knows best.” Ref: Aloloomenglish.net
Shaykh Hassan Bin Qaasim Ar-Raymee said concerning the permissibility of a Salafi student of knowledge giving a talk randomly (not pre-planned) in a masjid upon bidah if he passes by one.
He said, “There is nothing wrong with going there (to a mosque not upon the sunnah but upon bidah or just a cultural mosque not upon the truth nor belonging to a misguided sect/organisation), without it being organized (pre-planned in advance). Meaning it should be a lecture by one passing through (that locality) without (the lecture) being prearranged (in advanced).” Ref: Aloloomenglish.net
Posted by Abdul Kareem Ibn Ozzie