Sahih Bukhari Volume 9, Book 92, Hadith Number 465.

Narrated By ‘Abdullah Al Muzam : The Prophet said, “Perform (an optional) prayer before Maghrib prayer.” (He repeated it thrice) and the third time he said, “Whoever wants to offer it can do so,” lest the people should take it as a Sunna (tradition). (See Hadith No. 277, Vol. 2)