Musannaf Ibn Abi Shayba 34:272

Ubaydah narrated to us, from Dawud ibn Abi Hind, from al-Sha’bi, who said: The first time prayer was made obligatory, it was made obligatory as two rak'ahs [and] two rak'ahs. When the Prophet, may Allah bless him and grant him peace, came to Medina, he added two rak'ahs to every two rak'ahs, except for the Maghrib [prayer].