It is recommended for the fasting person to utter the intention, but uttering is not a condition for the validity of the intention; because the intention is valid as soon as the heart is determined on what one wants to perform of fasting, and this can be without utterance. However, uttering is recommended because it helps in bringing the intention to mind. Therefore, if one intends to fast while praying, the intention is valid, and the prayer is not invalidated unless the intention is uttered.