From Sahih Bukhari (Hadith 7368):
Narrated `Abdullah Al Muzam:The Prophet (ﷺ) 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)
Reference: Bukhari 7368