The prayer was originally prescribed as two units each. The travel prayer was kept as it was, while additional units were added to the prayer when at home.
أخبرنا قتيبة عن مالك عن صالح بن كيسان عن عروة
فرضت الصلاة ركعتين ركعتين فأقرت صلاة السفر وزيد في صلاة الحضر
Bukhari transmitted it, in the Book of Prayer, Chapter on how prayer was made obligatory during the Night Journey, no. 350, and Muslim, in the Book of Prayer of the Traveler, Chapter on the prayer of the traveler and its shortening, no. 1/685, from the hadith of Malik through this same route, and it is found in Muwatta (Yahya): 146/1.