Buratha is a holy place near Baghdad where an old church was build before Islam. Imam Ali (AS) while retuning from Nahrawan battle against the Kharijites , stopped at that place and prayed Salatul Jama'ah along with his army which was in tens of thousands. He ordered the monk to build a Masjid on that place. The Masjid was built on year 37 Hijri which is more than 109 years before the establishment of Baghdad by the Abbasids.
There are few narrations that a prophet was buried in Buratha, and narrations that Lady Maryam (Mary) deliver her son Jesus on a stone which is inside that place.
There is no doubt at all that Masjid Buratha is a sacred place that is why it was ordered by Imam Ali (AS) and he performed Salah in that place.