Sa'd bin Ubaydah (may Allah have mercy on him) narrates that Abdullah bin Umar (may Allah be pleased with them both) said: The fasting person has a choice until midday.
This has also been narrated through other routes as marfu' (from the Prophet (peace and blessings be upon him)), but its being marfu' is not sound.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8355