Hadith 4:220
Jarir narrated to us, from Layth, from Mujahid, who said: The fasting person has the choice [to break his fast] between [the start of the day] and midday. If he passes that, then he only has [the reward] in proportion to what remains of the day.
حدثنا جرير عن ليث عن مجاهد قال الصائم بالخيار ما بينه وبين نصف النهار فإذا جاوز ذلك فإنما له بقدر ما بقي من النهار