Is it true that the Prophet ﷺ promised the hellfire is ḥarām upon the one who first informs about the commencing of Ramaḍān?
There is no such ḥadīth to be found in the famous books.
It is quite possible that someone misquoted the following ḥadīth, then the misquoted version went viral:
«إذا دخل شهر رمضان فتحت أبواب السماء، وغلّقت أبواب جهنم، وسلسلت الشياطين»
“When the month of Ramaḍān enters, the doors of Paradise open, the doors of Jahannam are locked, and the devils are chained.”
However, this seems highly unlikely, as there are indentical messages going around on social media with regard to almost half the months of the hijrī calendar, including Muḥarram, Rabīʿ al-Awwal and Rajab.
As Mawlānā Muhammad Abasoomar writes:
“I haven’t seen any Hadith promising Jannah for merely being the first to break the news of anything.
‘Being the first’ to broadcast news/views is a trait synonymous to social media and not religion!
“In Islam we are encouraged to be the first to practice on whatever good we hear or learn. Therefore, before we pass on any authentic quote of Islam, we should be first to inculcate it [or at least intend to do so], before broadcasting it.”
We surely attest that Allāh ﷻ knows best.
Answered by Shahin-ur Rahman, Bedford.
Tuesday 12th Jumādā ’l-Ukhrā 1437 AH / 22nd March 2016 CE.
Checked and approved by Shaykh Dr. Abul Hasan Hussain Ahmed, London.
 Al-Bukhārī (1899).