From the (IS video)[http://news.yahoo.com/islamic-state-fighters-destroy-antiquities-iraq-video-124118694.html] is the statement: "The Prophet ordered us to get rid of statues and relics, and his companions did the same when they conquered countries after him," an unidentified man said in the video.
I'd like to know if this is historically true or simply a (false) religious justification for their actions.
One instances come to mind but is wasn't anything like ISIS and much more symbolic.
There is second one, but it's not really proven and has a likely chance of never happening.
What is the black meteorite stone imbedded in the Kaaba?
In terms of religious sources, Islam, like other monotheistic religions (particularly Judaism) is strongly and emphatically opposed to idol worship as a part of polytheism and actively encourages destroying statues ect. In terms of whether this act is in accordance to an Islamic tradition, I think most scholars would be interested mainly in the actual 'worship' of idols which would set it apart from this act.
Apart from the destruction of the idols around the Kaba by Muhammad, Umar as well as allegedly burning the library of Alexandria is also known for destroying a great amount of Persian books which contributed to the comparative lack of pre-Islamic Persian literature surviving to today. Given the Islamic writings on idols and statues and medieval Arab acceptance of Umar's destruction of books, I think it would be unsurprising if at least public relics and statues were desecrated in the Arab invasions of the Rashidun caliphs (it is almost impossible for me to say how much this was actually done however). However, this sort of vandalism was typical of warring armies in general, Alexander the great for example burned the library of Persepolis, the Romans and the library of Alexandria ect.
So while I don't think that ISIS is behaving at odds with the Islamic tradition left by the companions, it is equally acting in the tradition of conquerors and rulers getting rid of ideas and memories competing with their rule.