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The Hijrah
الهجرة
The Ansar pledged at Aqabah. The Prophet ﷺ left Makkah with Abu Bakr. They hid in the Cave. The Quran names the two: the second of two, when they were in the cave, and he said to his companion, do not grieve, Allah is with us. Suraqah followed and could not. Madinah received him ﷺ. The mosque was built. The brotherhood was made. The qiblah later turned to the Kabah.
From the Quran
From the Sunnah
The Hijrah is the date of this ummah. Everything after is counted from it.