Courtesy of Dar us-Sunnah (original). This is an important fatwa as it addresses an issue that might be common in many lands where Salafis do not have mosques and may enter into cooperation with mosques that allow many different types of speakers to lecture and address the people.

The question:

We have some masajid in Britain that bring a person who is upon the methodology of the Sunnah, in order that he may teach or lecture people about Tawheed. Then on following day, they will bring a Sufi or Ash'ari who will teach the people Sufi or Ash'ari [aqeedah]. Is it permissible for this Sunni preacher to teach in such a masjid?

Shaykh Salih al-Fawzaan:

No, If they [the masajid] mix between him [the Sunni] and an innovator or a deviant, it is not permissible for him [to teach there]. This is because, they are using him as a disguise. They say [to the people], 'We teach Tawheed and we listen to lectures in Tawheed!' This is not correct, since this is mixing the truth and falsehood. [The Sunni] should place a condition on them and say, 'I will teach or deliver lectures for you on a condition that you do not bring individuals whom are in opposition to that which I say [teach] to you and call you to.'