Sahih Bukhari
Holding Fast to the Quran and Sunnah
Volume: 009 / Book Number: 092 / Hadith Number: 465
Narrated '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)