It is narrated from Sa'id bin al-Musayyab that he said: "Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) gathered the people and consulted them about the day from which the date (calendar) should be written. Sayyiduna Ali bin Abi Talib (may Allah be pleased with him) said: 'From the day on which the Messenger of Allah (peace and blessings be upon him) emigrated and left the land of polytheism.' So Sayyiduna Umar (may Allah be pleased with him) acted upon that and fixed the calendar accordingly." This hadith has an authentic (sahih) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.