Hisham bin Urwah (may Allah have mercy on him) narrates from his father (may Allah have mercy on him) that Sayyidah Aishah (may Allah be pleased with her) used to pray four rak'ahs while travelling. I said to her: What if you prayed two rak'ahs? She said: O my nephew, it is not hard for me.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 5430