In one narration from Sayyidah Aishah (may Allah be pleased with her) it is reported that she said: In Makkah the prayer was made obligatory as two rak'ahs and two; when the Prophet (peace and blessings be upon him) came to Madinah, two rak'ahs were added to every two rak'ahs. After that the narrator narrated the rest as before. Ahmad recorded both narrations.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2932