Sayyidah Aisha Siddiqa (may Allah be pleased with her) narrates: When the prayer was made obligatory, two rak'ahs were prescribed for both residence and travel. Then, the prayer during residence was increased, and the prayer during travel was kept in its original state.
Hadith Referenceصحیح ابن حبان / كتاب الصلاة / 2737
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح: ق - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح