Urwah bin al-Zubayr (may Allah have mercy on him) narrates from Sayyidah Aishah (may Allah be pleased with her) that when the prayer was made obligatory, it was two rak'ahs each, both when travelling and when resident. Then the prayer of travel was kept at two rak'ahs, and the prayer of the resident was completed to four rak'ahs. He says: I told this to Umar bin Abd al-Aziz (may Allah have mercy on him), and he said: But Urwah told me that Sayyidah Aishah (may Allah be pleased with her) used to pray four rak'ahs while travelling. One day I found Urwah (may Allah have mercy on him) with him. I said: Tell me about Aishah (may Allah be pleased with her)! He narrated it to me just as Umar bin Abd al-Aziz (may Allah have mercy on him) had narrated it.
Umar bin Abd al-Aziz (may Allah have mercy on him) said: Did you not narrate that she used to pray four rak'ahs while travelling? He said: Yes indeed.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5431