Q- Is this Hadith authentic? إن لله مدائن لا يدخلها إلا من صام رجب. Allah Ta’ala has special cities [in the Hereafter], only those who would fast in Rajab will enter them.?
This narration appears in Musnadul Firdaws as part of a longer narration. However, the chain contains a narrator who has been a liar by some Muhaddithun. Therefore, the Muhaddithun have cited this narration in their books on fabricated Hadiths. (Musnadul Firdaws: 247, Mizanul I’tidal, vol. 1 pg. 259, number: 959, Dhaylul La-Alil Masnu’ah, Hadith: 753, vol. 2 pg. 617, Tanzihush Shari’ah, vol. 2 pg. 333. Also see Al Adab Fi Rajab, pg. 38)
The narration is not suitable to quote.
And Allah Ta’ala knows best.
https://hadithanswers.com/cities-in-the-hereafter-reserved-for-those-who-fast-in-rajab/