Abu Dawud was asked about offering prayer at night in two rak'ahs. He said: They may be two if you like and four if you like.
In-book : Book 5, Hadith 47
English translation : Book 5, Hadith 1291
5. Prayer (Kitab Al-Salat): Voluntary Prayers
باب فِي صَلاَةِ النَّهَارِ