Is verbal utterance necessary for an oath to be valid or will an oath be formed if a firm intention is made in the heart without saying it loudly with the mouth and tongue?
بسم الله الرحمن الرحيم
(Fatwa: 1230/902/SN=11/1439)
Yes! It is mandatory for the validity of oath to be uttered verbally. According to Shariah, oath does not come into effect merely by making intention in the heart. In such case if one goes against it then according to Shariah there is no kaffarah. (See: Shami 5:473, pub. Zakaria)
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband