Abu Dawud Book 2, Hadith Number 1277.
Chapter : Prayer before the sunset prayer.
Narated By Anas b. Malik : I offered two rak’ahs of prayer before the sunset prayer (i.e. obligatory) during the time of the Apostle of Allah (PBUH). I (the narrator al-Mukhtar b. Fulful) asked Anas: Did the Apostle of Allah (PBUH) see you? He replied: Yes, but he neither commanded us nor forbade us (to do so).