A number of our people have narrated from Ahmad ibn Muhammad from Ahmad ibn Muhammad ibn abu Nasr from Hisham ibn Salim from Sa‘d who has said the following: “Once, we eight people were in the presence of abu Ja’far (عليه السلام). We spoke of Ramadan. The Imam said, ‘You must not say, “This is Ramadan or Ramadan is gone or Ramadan has come.” Ramadan is one of the names of Allah, the Most Majestic, the Most Glorious, which does not come or go. What comes or goes is what is not durable. You instead should say, “The month of Ramadan.” The word “month” in this case is used in a possessive case in relation to the noun, the “name,” the name that is the name of Allah, the Most Majestic, the Most Glorious. This month is the one when the Holy Quran was revealed, and He has made it a festival and an example."
Al-Kāfi, Volume 4, the book of fasting, chapter of the prohibition of saying 'ramadhan' instead of the month of ramadhan.