Combining two dead People in one Grave due to necessity

Question :

It happened that a baby girl died who was six months old and she was buried with a baby boy who died from a miscarriage when he was in his sixth month in his mother's womb. Is this permissible or not. If not, then what is the ruling of those who buried them in one grave?

Answer:

What is legislated is that each deceased person be buried in a grave alone. This is the Sunnah that the Muslims have acted upon since the time of the Prophet until our time now. However, if there is a need that calls for burying two or more people in a single grave, then there is no harm in that. Verily, it is confirmed in the Two Sahihs and other than them that the Prophet used to put two or three men from the martyrs of the battle of Uhud in a single grave if there was a need for that. Concerning this baby girl and this miscarried child that were buried together in one grave, it is not obligatory to dig them up now because it is too late. Whoever buried them in a single grave due to ignorance concerning that, then there is no sin on him. However, it is necessary that everyone who does an action from the acts of worship or other than them know the limits of Allâh concerning that action before he proceeds forth with it. This is so that he does not fall into that which should be avoided Islamically.

Source:
Ash-Shaykh Ibn 'Uthaimin
Fatawa Islamiyah, Vol. 3 Pages 76-77