Silsilat al-Ahadith al-Sahihah

It is narrated from Sayyidah Aisha (may Allah be pleased with her), she says: In the beginning, the prayers were made obligatory as two rak'ahs each. When the Prophet (peace and blessings be upon him) came to Madinah, an equal addition was made to each prayer, except for the Maghrib prayer, as it makes the prayers of the day odd (i.e., an odd number), and except for the Fajr prayer, in which a long recitation is done. And when the Prophet (peace and blessings be upon him) traveled, he would perform the prayers according to the original (two rak'ahs) manner.

Sayyidah Aisha (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) used to perform two rak‘ahs of obligatory prayer in Makkah. When he migrated to Madinah, the prayers of four and three rak‘ahs were made obligatory. He (peace and blessings be upon him) acted upon this new command, and the two rak‘ahs that were performed in Makkah were established as the complete prayer for the traveler.