Answered by Mufti Mohammed Tosir Miah
Will it be permissible for a person who enters the toilet to read the dua inside the toilet?
It will be permissible for him to read it in his heart and preferable not to read it verbally on his tongue. (Asr Hadhir Ke Masaail p.195 v.1)
Only Allah Knows Best
Mohammed Tosir Miah
Darul Ifta Birmingham