There is no basis for this hadeeth in the books of Sunnah and reports, and no narration is known for it with any of the scholars. Similarly, we could not find it in the books which deal with false and fabricated hadeeths.
Rather we found it in some of the books of the Shiah that are filled with fabricated reports that are narrated without any isnaad or proof. It was mentioned by Ibn Tawoos - Ali ibn Moosa ibn Jafar (d. 664 AH) - in his book Iqbaal al-Amaal (3/216). But we did not find any basis for this hadeeth in any earlier book of the Shiah. Ibn Tawoos mentioned it in a comment without any isnaad, and said:
Chapter on what we have mentioned about the virtue of seeking forgiveness, repeating Laa ilaaha ill-Allah and repenting in the month of Rajab. We found this narrated from the Prophet (may Allah send blessings upon him and his family), that he said: Whoever says in Rajab I ask Allah for forgiveness, there is no God but He alone, with no partner or associate, and I repent to Him' one hundred times, and ends it with charity, Allah will decree for him mercy and forgiveness. And whoever says it four hundred times, Allah will decree for him the reward of one hundred martyrs, and when he meets Allah on the Day of Resurrection, He will say to him: You affirmed My sovereignty, so wish for whatever you want so that I may give it to you, for there is none who prevails but Me.
It was transmitted from him in some of the other books, such as Wasaa'il ash-Shiah (10/484) by al-Hurr al-Aamili (d. 1104 AH) and others.
Thus the signs of fabrication in this hadeeth become clear:
The hadeeth is devoid of any isnaad.
It is only the books of the Raafidah that mention this hadeeth. From their books the hadeeth has become well known in some Internet chat rooms and websites. Hence it is essential to be cautious about many of the hadeeths that are quoted in chat rooms when their sources are the fabricated books of the Raafidis.
The hadeeth has to do with the virtues of the month of Rajab. We must be cautious about all reports that speak of this matter, because there are so many fabricated reports concerning it that some of the scholars wrote books dealing specifically with that, such as al-Haafiz Ibn Hajar (may Allah have mercy on him) in his book Tabyeen al-Ajab bima wurida fi Fadl Rajab. He (may Allah have mercy on him) said:
No saheeh hadeeth that is fit to be used as evidence has been narrated concerning the virtue of the month of Rajab or of fasting that month or any specific part of it, or praying qiyaam al-layl specifically during that month. This has been stated categorically before me by Imam Abu Ismaaeel al-Harawi al-Haafiz, and we have narrated it from him with a saheeh isnaad as we also narrated it from others. But it is well known that the scholars are deviant in narrating hadeeths about virtues, even if they are daeef, so long as they are not fabricated. Nevertheless we should stipulate that the one who does that should believe that the hadeeth is daeef and he should not make it well-known, lest people act upon a da eef hadeeth and prescribe something that is not prescribed, or lest some of the ignorant see that and think that it is a saheeh Sunnah.
End quote from Tabyeen al-Ajab (p. 11)
Exaggeration about the reward, whereby a small deed in the month of Rajab is said to bring the reward of one hundred martyrs and more. Such as it is not narrated in the saheeh reports.
And Allah knows best.
Reproduced from Islam QA