Prohibition from uttering his special name has its roots in traditions. In Kafi, book of Al-Hujjah, Kulaini has opened a chapter named باب في النهي عن الاسم (chapter concerning prohibition from uttering name). He has narrated four traditions in it and Allamah Majlisi, while commenting on these four traditions has authenticated two of them.
Mohaddes Nuri has brought 13 traditions in second chapter of his book“Najm-e-Saqeb” which all show that it’s impermissible to utter the special name. Here, we shall set forth only one tradition:
Aban-ibn-Salt narrates from Imam Reza (a.s.) that he was asked about the“Qaem” . Imam (a.s.) replied:“His body wouldn’t be seen and his name wouldn’t be uttered.” 
The certain and decisive point is that utterance of Hazrat’s special name was prohibited till the end of“short concealment” considering the requirements of that period. In the past, the immaculate Imams refrained from disclosing his name except to a few special Shias and content themselves with various titles and agnomens. However, after the start of greater concealment, some scholars and jurisprudents have considered it permissible to utter Hazrat’s name although they consider its abandonment a precautionary measure as a form of respect for those set of traditions that prohibit it.
Today, our duty is to act upon the traditions, and the views of majority of the jurisprudents and scholars of Hadith. Although as per a few, the philosophy for prohibition of uttering name has ceased; yet the endless interests and harms stated in divine commandments and man’s meager knowledge and lack of complete dominance over all of them necessitates us to take precautionary measures and not to utter Imam’s name.
For this reason, we too follow the steps of high-ranking scholars on Hadith and refrain from uttering Imam’s special name “م ح م د ” and remember and call him by various titles and seek his help by saying:
“O chief! distress has afflicted us and our family and we have brought scanty money, so give us full measure and be charitable to us; surely Allah rewards the charitable.” (Qur’an, 12:88)