Letters would come to him, and he would not know when they were written and when they were sent. Therefore, he (رضي الله عنه) set the calendar to make it begin from the month of al-Muḥarram.
Al-ʿAllāmah Ṣāliḥ ibn Fawzān al-Fawzān
Shaykh Ṣāliḥ al-Fawzān (حفظه الله) said:
As for the beginning of the year [starting the Islamic calendar with al-Muḥarram], then it is an affair that was adopted in the era of ʿUmar [ibn al-Khaṭṭāb, the second rightly guided Khalīfah] (رضي الله عنه) because of the letters that would come to him from his workers.1
Letters would come to him, and he would not know when...