Abu al-Duha (may Allah have mercy on him) narrates that a man said to Sayyiduna Ibn Abbas (may Allah be pleased with them both): When should I stop eating the pre-dawn meal (suhur)? A man said: When you begin to doubt. Then Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Eat as long as you are in doubt, until it becomes clear to you.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8038