Abu Dawud Book 2, Hadith Number 1295.

Chapter : Where should the two rakahs of the sunnah prayer of the sunset prayer be offered.

Narated By Ka’b ibn Ujrah : The Prophet (PBUH) came to the mosque of Banu Abdul Ashhal. He prayed the sunset prayer there. When they finished the prayer, he saw them praying the supererogatory prayer after it. He said: This is the prayer to be offered in the houses.