Sahih Bukhari
Merits of the Helpers in Madinah (Ansaar)
Volume: 005 / Book Number: 058 / Hadith Number: 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).