Sahih Bukhari | Volume 9 | 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)