Abu Abd al-Rahman (may Allah be pleased with him) says that it occurred to Hudhayfah (may Allah be pleased with him) to fast after the sun had passed its zenith, so he fasted.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7920
Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: As long as a person has not eaten or drunk anything, he has the choice.
Imam al-Shafi'i (may Allah have mercy on him) says: They (meaning the people of Iraq) do not hold this view; rather, they say that unless he makes the intention to fast before the sun passes its zenith, his fast will not be valid. But we say that in a voluntary fast he may make the intention whenever he wishes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7921