Hadith 4:173
Ibn ‘Uyaynah narrated to us, from Ibn Abi Najih, from Mujahid, regarding the one who eats the pre-dawn meal (suhur) while he thinks that it is still night, and it becomes clear that he ate the pre-dawn meal after the dawn had risen: he should complete his fast.
حدثنا بن عيينة عن بن أبي نجيح عن مجاهد فيمن تسحر وهو يرى ان عليه ليلا فبان انه تسحر وقد طلع الفجر فليتم صيامه