Q:When is the intention to fast Ramadan made?
A:Shaykh Ahmed Al-Najmi rahimahullaah said:
The intention for any obligatory fast should be made (in the night) before Fajr (prayer). As for a voluntary fast then the intention can be made even after sunrise so long as the person hasn’t eaten or drunk then his fast is correct.
{Translator’s note: The scholars have mentioned that it is sufficient to do one intention for the whole of Ramadan rather than do it every night and Shaykh Ubayd al-Jaabiri adds unless one travels or falls sick (hence breaking their fast) then they repeat their intention once they resume the fast}
(Fataawa no. 268 Shaykh Ahmed Al-Najmi rahimahullaah)