Ash-Shaykh Bin Baaz, rahimahullah was asked the following question:

إذا نوى الإنسان الصيام بعد صلاة الفجر، فهل يصح له ذلك؟

Question:

If a person intends to fast after Salaatul Fajr, would that be valid for him?

إذا كان نافلة ولم يأكل شيئاً ولم يتعاطى مفطراً صح، إذا نوى الصوم بعد الفجر أوالضحى أو بعد ذلك, ويكون له الأجر من حين نوى، يكون أجر الصوم يكون له من حين نوى، هذا في النافلة خاصة. أما الفرض لا،

Answer:

If it is a volunteer fast and the person hasn’t ate anything nor has he took what will break his fast, then it is valid. If he intends to fast after dawn or after sunrise or (even) after that, he will get the reward from the time he begins his intention. (But pay attention to the fact that) this is specifically in regards to the volunteer fast. As for the obligatory fast, then no (i.e the intention for fasting must start before dawn)… ”

[Reference]

Translated by

AbdulFattaah bin Uthman
Abu Fajr