When a person dies why does one have to cover the mouth of the dead like with a string, is there any evidence for that?

In the name of Allah, the most beneficent, the most merciful.

Answer:

When a person passes away the eyes and mouth of the deceased should be closed, its chin tied, its hands and feet be straightened. (As these will be difficult to do once the body is cold and stiff.) The closing of the mouth and tying of the jaws will stop anything entering the mouth of the deceased. This will make the deceased more presentable and not make his appearance fearsome or disliked. (Raddul Muhtar, Vol 1, P630)
Only Allah Knows Best

Sayeedur Rahman

Darul Ifta, Birmingham