Volume 5 · Book 58 · Hadith 272
Narrated 'Aisha: Originally, two Rakat were prescribed in every prayer. When the Prophet migrated (to Medina) four Rakat were enjoined, while the journey prayer remained unchanged(i.e. two Rakat).
— Sahih Bukhari, Volume 5, Book 58: Merits of the Helpers in Madinah (Ansaar), Hadith #272