Whenever one utters the name or title of Imam (as) one should stand up in respect, as had been the practice of the Shiah community. A report says that one day the name of Imam Mahdi (as) was mentioned in gathering of Imam Sadiq (as). When the name was uttered, Imam Sadiq (as) stood up in respect. (Beharul Anwar volume 44)
That standing up in respect is recommended (Mustahab) is proved from this above tradition, but there are some occasions when it is obligatory to do so. For example, when the name of Imam (as) is uttered and all the people stand up. Anyone who continues to sit without any valid excuse has disrespected Imam (as). And there is no doubt that disrespect towards Imam (as) is haraam (forbidden).