Prayer was originally prescribed as two units, both at home and while traveling. The traveler's prayer was left as it was, and the prayer performed at home was increased.
و حدثني عن مالك عن صالح بن كيسان عن عروة بن الزبير
فرضت الصلاة ركعتين ركعتين في الحضر والسفر فأقرت صلاة السفر وزيد في صلاة الحضر