Will an oath be valid if it is made in the heart without uttering it?
بسم الله الرحمن الرحيم
(Fatwa: 1049/1062/M=9/1438)
It is necessary that the words of oath are uttered verbally. The oath shall not take place merely by saying in the heart.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband