DAWAH TO MUSLIM IS SUNNAH OR BIDAH
1.From Quranic Ayats and Ahadith it is quiet clear that Dawah is essential duty of Muslims.It is for Both Muslims and Nonmuslims and also for Daee himself. BUT..............
For quranic Ayats and Ahadith on this see the link http://tablighijamaattruth.blogspot.in/p/dawah-for-both-muslim-non-muslim.html
Some people Propagate that Dawah to Muslims a Bidah.
These people may be.
1.Having little understanding of Quran and Ahadith and the Life of Prophet and Sahaba.
2.It is also a possibility that these people simply want to confuse the muslims for some wordly gain .(Allah nows the best)