Abu Dawud Book 2, Hadith Number 1276.
Chapter : Prayer before the sunset prayer.
Narated By ‘Abd Allah al-Muzani : The Apostle of Allah (PBUH) said: Pray two rak’ahs before the sunset prayer. He then said (again): Pray two rak’ahs before sunset prayer; it applies to those who wish to do so. That was because he feared that the people might take it as a sunnah.