Reference: Fataawa Ramadhaan – Volume 1, Page 173, Fatwa No.111
Fataawa as-Siyaam libni Jibreen – Page 39
Question: What is the ruling regarding making the intention vocally, such as when some people make the intention to fast [by saying]; “O Allaah, I have made the intention to fast”?
Response: The place of the intention is in the heart and it is not permissible to vocally make the intention either for the prayer, or the fast, or the purification or other than these.