It is narrated from Sayyiduna Abdullah al-Muzani (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Pray two rak'ahs before Maghrib." Then he said: "Whoever of you wishes may pray two rak'ahs before Maghrib." He said these words fearing that people would take it as a Sunnah.
(b) In the narration of Abd al-Warith it is that he (peace and blessings be upon him) used the words "whoever wishes" the third time so that people would not take it as a Sunnah.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4488