Establishment of the Hijri Calendar

The Great Jurist of Madinah, Imam Sa’id ibn al-Musayyib {Allah send His Divine Mercy upon him} said:

The first one to establish the {Hijri} calendar was ‘Umar ibn al-Khattab {Allah be pleased with him} two and a half years into his caliphate. He reached a decision in consultation with ‘Ali Ibn Abi Talib {Allah be well pleased with him}

{Tarikh al-Islam ~ The History of Islam by Imam ad-Dhahabi}