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
Habib bin Abi Thabit (may Allah have mercy on him) said that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) sent two men to look out for the dawn. One of them said: It is morning for you; and the other said: It is not morning for you. He said: You two disagree; bring me water.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8039