Purity: Burial Without Valid Prayer
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 596:
If a corpse is intentionally, forgetfully, or for some other reason buried without the prayer being performed for it, or if after burying a corpse it becomes known that the prayer performed for it was invalid, it is not permitted to exhume the body to perform the prayer for it. However, there is no problem if, before the body decomposes, the prayer is performed at the grave-side with the intention of rajāʾ while observing the conditions mentioned earlier for this prayer.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)