Praise be to Allah.
Fabricated hadith about the month of Rabi` al-Awwal
We do not know of any source for the hadith which reads: “Whoever offers congratulations to people for this blessed month, the Fire will be forbidden for him”, and it is clearly fabricated.
Hence it is not permissible to attribute it to the Prophet (blessings and peace of Allah be upon him), because that comes under the heading of telling lies against him , and telling lies against him is haraam and is a major sin.
Warning against attributing false hadiths to the Prophet
The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever narrates a hadith from me knowing that it is false is one of the liars.” (Narrated by Muslim in the Introduction to as-Sahih, 1/7).
An-Nawawi (may Allah have mercy on him) said:
“This is a stern warning against lying, which indicates that whoever thinks it most likely that what he is narrating is false but still narrates it is a liar. How can he not be a liar, when he is narrating something that did not happen?” (Sharh Sahih Muslim, 1/65)
What is mentioned in this (so-called) hadith about the Fire being prohibited to a person for simply uttering this greeting comes under the heading of reckless talk and exaggeration which indicate that a hadith is fabricated and false.
Ibn al-Qayyim (may Allah have mercy on him) said: “Fabricated hadiths sound strange , are poorly-worded and contain exaggeration, which clearly indicates that they are fabricated and are lies told against the Messenger of Allah (blessings and peace of Allah be upon him).” (Al-Manar al-Muneef, p. 50)
And Allah knows best.