Sahih Al Bukhari Hadith Number 1183

Narrated `Abdullah Al-Muzni: The Prophet (peace be upon him) said, “Pray before the Maghrib (compulsory) prayer.” He (said it thrice) and in the third time, he said, “Whoever wants to offer it can do so.” He said so because he did not like the people to take it as a tradition.

19  Prayer at Night (Tahajjud)
Hadith number 1183
Volume no 2
Chapter: 35 (The (optional) salat before the Maghrib prayers)