Hadith 29:235
Muhammad ibn Abd Allah al-Asadi narrated to us, Hayyan narrated to us, from Mujalid, from al-Sha’bi, who said: Abu Musa wrote to Umar, "Letters come to us, but we do not know their dates, so establish a date." Umar consulted the companions of the Prophet, may Allah bless him and grant him peace. Some of them said, "Date it from the mission (bi'tha) of the Messenger of Allah, may Allah bless him and grant him peace." Others said, "Date it from the death of the Messenger of Allah, may Allah bless him and grant him peace." Umar said, "Date it from the migration (hijra) of the Messenger of Allah, may Allah bless him and grant him peace, for the migration of the Messenger of Allah, may Allah bless him and grant him peace, is a separator between truth and falsehood." So they established the date.
حدثنا محمد بن عبد الله الأسدي حدثنا حيان عن مجالد عن الشعبي قال كتب أبو موسى إلى عمر أنه يأتينا كتب ما نعرف تأريخها فأرخ فاستشار أصحاب النبي صلى الله عليه وسلم فقال بعضهم أرخ لمبعث رسول الله صلى الله عليه وسلم وقال بعضهم أرخ لموت رسول الله صلى الله عليه وسلم فقال عمر أرخ لمهاجر رسول الله صلى الله عليه وسلم فان مهاجر رسول الله صلى الله عليه وسلم فرق بين الحق والباطل فأرخ