What is the ruling on fasting the entire month of Rajab? Some people claim that this is not permissible and that whoever does so commits a sin. Is this claim correct?

Fasting the entire month of Rajab is permissible in Islamic law, and there is no objection nor sin upon one who does so. This ruling is based on the generality of the hadiths that speak of the virtue of voluntary fasting. Moreover, none of the recognized scholars of the Muslim Ummah have reported that fasting the whole of Rajab falls under what is considered disliked (makruh).[1]

And Allah Almighty knows best.