The story of the formation of the Hijri calendar by Umar b. al-Khattab

A man from among the Muslims came from the land of Yemen and said to ʿUmar: “In Yemen I have seen something they call ‘dating’ (the calendar): they write ‘from such-and-such a year and such-and-such a month.’” ʿUmar said: “This is good—so date your documents.”

When he had resolved to establish a dating system, he consulted them. Some said: “From the Prophet’s ﷺ birth.” Others said: “From the beginning of his mission.” Others said: “From the time he set out as an emigrant from Makkah.” And one said: “From the time of his death, when he passed away.” Some said: “Date it from his departure from Makkah to Medinah.”

Then he said: “With what shall we begin, so that we make it the first of the year?” They said: “Rajab, for the people of the pre-Islamic period used to venerate it.” Others said: “The month of Ramadan.” Some said: “Dhu al-Hijjah.” Others said: “The month in which he departed from Makkah.” Others said: “The month in which he arrived.” Then ʿUthmān said: “Date it from Muharram as the first of the year. It is a sacred month; it is the first of the months in the reckoning; and it is when people return from the Hajj. So make Muharram the first of the year.” That was in the year 17 AH, and it is also said: 16 AH, in Rabiʿ al-Awwal.  

"أَنَّ رَجُلًا مِنَ الْمُسْلِمِينَ قَدِمَ "مِنْ" أَرْضِ اليَمَنِ فَقَالَ لِعُمَرَ: رَأَيْتُ باليَمَنِ شَيْئًا يُسَمُّونَهُ التَّارِيخَ، يَكْتُبُونَ مِنْ عَامِ كَذَا، وَشَهْرِ كَذَا، فَقَالَ عُمَرُ: إِنَّ هَذَا لَحَسَنٌ فَأرِّخُوا،

فَلَمَّا أجْمَعَ "عَلَى" أَنْ يُؤَرِّخَ شَاوَرَهُمْ، فَقَالَ قَوْمٌ: بِمَوْلِدِ النَّبِىِّ ﷺ ، وَقَالَ قَوْمٌ: بِالمَبْعَثِ، وَقَالَ قَوْمٌ: حِينَ خَرَجَ مُهَاجِرًا مِنْ مَكَّةَ، وَقَالَ قَائِلٌ لوفاته حِينَ تُوفِّى، فَقَالَ قَوْمٌ: أَرِّخُوا خُرُوجَهُ مِنْ مَكَّةَ إِلَى الْمَدِينَةِ،

ثُمَّ قَالَ: بِأىِّ شَىْءٍ نَبْدَأُ فَنُصَيِّرُهُ أَوَّلَ السَّنَةِ؟ فَقَالُوا: رَجَبٌ فَإنَّ أَهلَ الجَاهِلِيَّة كَانُوا يعظمونه، وَقَالَ آخَرُونَ: شَهْرُ رَمَضَانَ، وَقَالَ بَعْضُهُمْ: ذُو الحِجَّةِ، وَقَالَ آخَرُونَ: الشَّهْرُ الَّذِى خَرَجَ مِنْ مَكَّةَ، وَقَالَ آخَرُونَ: الشَّهْرُ الَّذِى قَدِمَ فِيهِ، فَقَالَ عُثْمَانُ: أرِّخوا مِنَ المُحَرَّم أَوَّلَ السَّنَةِ، وَهُوَ شَهرٌ حَرَامٌ، وَهُوَ أَوَّلُ الشُّهُورِ في العِدَّةِ، وَهُوَ مُنْصَرَفُ النَّاسِ مِنَ الحَجِّ، فَصَيِّرُوا أَوَّلَ السَّنَةِ المُحَرَّمَ، وَكَانَ ذَلِكَ سَنَةَ سَبْعَ عَشْرَةَ، وَيُقَالُ: سِتَّ عَشْرَةَ في رَبِيعٍ الأَوَّلِ".