Shaikh Rabīʿ ibn Hādī al-Madkhalī, may Allāh protect him, said:
As for asking Allāh to have mercy upon people of innovation [by saying raḥimahullah–may Allāh have mercy on him–and the like], then indeed, it is permissible to ask Allāh to have mercy on them.
This is something the Righteous Predecessors were upon, and from them was Aḥmad ibn Ḥanbal. Texts from the Book of Allāh, All-Great is He and All-High, and from the Sunnah of the Messenger of Allāh ﷺ are evidence for that.
The one who contests this is ignorant and astray.
May Allāh increase in His praises for our Prophet Muḥammad, his family, and his companions; and may He protect them from every harmful thing.
Translated by Mikail ibn Mahboob Ariff Ariff