Assalaamu alaykum. This message has been spread by a lot of people these days. “Spread the news of Ramadan.” “The Prophet, sallallaahu 'alayhi wa sallam, said that if one informs another about Ramadan first, then hellfire will become haram for that individual.” Please, can you let me know if this hadith is authentic or not. May Allaah reward you.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger. 

We have not come across, in the books of the scholars that we have at hand – despite their abundant number – a hadeeth with the meaning that you mentioned. Nevertheless, there is a hadeeth that indicates the permissibility of giving glad tidings (telling people to make them rejoice) at the occasion of the coming of Ramadan; Abu Hurayrah narrated that the Prophet said to his Companions, “There has come to you the month of Ramadan, which Allaah has enjoined you to fast. In this month, the gates of Paradise are opened and the gates of Hellfire are closed. The devils are chained in this month, and there is a night in it which is better than a thousand months of worship: whoever is deprived of its goodness is indeed deprived.” [Ahmad - Shu'ayb al-Arnaa’oot graded it saheeh (sound)]

Allaah knows best.