The prayer was originally prescribed as two units, two units, both at home and while traveling. The traveler's prayer was left as it was, and additional units were added to the prayer performed at home.
حدثنا يحيى بن يحيى قال قرأت على مالك عن صالح بن كيسان عن عروة بن الزبير
فرضت الصلاة ركعتين ركعتين في الحضر والسفر فأقرت صلاة السفر وزيد في صلاة الحضر