Teaching in the masaajid of the people of the sunnah is (what's) desired. However, if he goes to (the masaajid of the people of Bid'ah) and gives a word (speaks) and clarifies in it the Sunnah as well as (explains) the danger of Bid'ah, indeed this is good; this is as long as he is able (allowed) to speak a word clarifying the Sunnah. As for him speaking on something which will distort (that which is intended for him to explain) or something which they (the people of Bid'ah) want (him to speak about), then (no), don't go.