I read that there are some narrations in Tahdheeb al-Ahkam whereby our Imams (as) prohibited construction over graves. Is this true? If so, what is the ruling of the tombstones and shrines over the graves of our Imams?
"…Said those who prevailed in the matter, "We will surely take [for ourselves] over them a masjid." Quran 18:21
We can understand from this ayah, about the people of the cave, the permissibility of building masajid at burial sights of respected individuals.
Also, refer to Kamil al Ziyarat for many narrations about the significance of visiting Imam Hussain's (as) shrine.