Abdul Aziz bin Hakim al-Hadrami said: I heard Sayyiduna Ibn Umar (may Allah be pleased with them both) say: If I were to fast the whole year, I would still break the fast on the day of doubt about Ramadan.
Imam al-Thawri (may Allah have mercy on him) narrated from Abdul Aziz that he said: I saw Sayyiduna Ibn Umar (may Allah be pleased with them both) ordering a man to break the fast on that very day about which there is doubt.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7958