It is narrated from Sayyidah Aishah (may Allah be pleased with her) that in Makkah the prayer was made obligatory upon the Prophet (peace and blessings be upon him) as two rak'ahs each; when he left for Madinah, four were made obligatory, and the prayer of travel was kept at two rak'ahs.
This specification is something Ma'mar bin Rashid alone narrated from al-Zuhri; all the other trustworthy narrators reported it without qualification.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1696