Ayyub (may Allah have mercy on him) said that Umar bin Abdul Aziz (may Allah have mercy on him) wrote to the people of Basrah that this had reached us from the Messenger of Allah (peace and blessings be upon him). Then he narrated a hadith like that of Ibn Umar (may Allah be pleased with them both) and said: "It is good that the days be estimated: we saw the crescent of Sha'ban on such-and-such a day, so Ramadan falls on such-and-such a day. But if the crescent is seen before that, then act according to it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7929