Narrated Sahl bin Sa`d: The Prophet's companions did not take as a starting date for the Muslim calendar, the day, the Prophet had been sent as an Apostle or the day of his death, but the day of his arrival at Medina.
Hadith Referenceصحيح البخاري / كتاب مناقب الأنصار / 3934
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة
Narrated `Aisha: Originally, two rak`at were prescribed in every prayer. When the Prophet migrated (to Medina) four rak`at were enjoined, while the journey prayer remained unchanged(i.e. two rak`at).
Hadith Referenceصحيح البخاري / كتاب مناقب الأنصار / 3935
Hadith Gradingمحدثین:أحاديث صحيح البخاريّ كلّها صحيحة