Imam Al Uthaymeen said:
“All of the narrations which have come in regards to the virtues of praying in (the month of) Rajab or the virtues of fasting in Rajab, than all of them are very weak narrations. Rather, some of the scholars said they are fabricated and a lie upon the prophet صلى الله عليه و سلم. Thus, it’s not permissible for anyone to rely upon these narrations.
It’s not permissible for anyone to rely upon it and specify Rajab with fasting or praying because that is an innovation and the prophet صلى الله عليه و سلم said:
“Every innovation is misguidance and every misguidance is in the fire”
He also said:
“It’s not permissible for anyone to specify a specific time, or place to (perform) an act of worship which neither Allah nor his messenger specified, because we worship in accordance to the legislation of Allah (and) not our desires, inclinations nor emotions.
Indeed that which is obligatory upon us is that we say ‘we hear and we obey’, we do that which Allah commanded with, and leave off that which Allah prohibited, and we don’t legislate for ourselves acts of worship which Allah nor his messenger didn’t legislate.
Oh you Muslim, indeed In that which has come in the book of Allah and in that which has been authentically reported from the prophet صلى الله عليه و سلم from righteous actions suffices from that which has come in weak, fabricated narrations which are a lie upon the prophet صلى الله عليه و سلم.
If a person worships Allah with that which has been affirmed that it is from the legislation of Allah, then indeed he has worshipped Allah upon clear insight, seeking the reward of Allah and fearing his punishment….
Majid Jawed Al-Afghanee