Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1276
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.