All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
The Prophetic narration which reads "Hasten to perform the two Rak’ahs of Sunnah after the Maghrib obligatory prayer" is reported by Al-Bayhaqi and others but its chain of narrators was classified as weak by many scholars, like Al-Munaawi, As-Sakhaawi, Al-‘Ajlooni and Al-Albaani . This narration also contradicts the authentic narrations which prove the dislikeability of performing the supererogatory prayer straight after the obligatory prayer.
After investigation, we have not come across any statement of Ibn Hajar about this precise narration. However, he stated the legislated mention of Allaah after the obligatory prayer, and said: “This is a controversial issue, whether or not one should busy himself with the legislated mention of Allaah before performing the supererogatory prayer and then perform the supererogatory prayer –and this is the opinion of most scholars –but the view of the Hanafi School of jurisprudence, is that one should start with the supererogatory prayer...”
However, the authentic narrations prove that one should first recite the legislated mention of Allaah after the obligatory prayer, and he should not abandon this mention just for the above narration which is proven to be Dha’eef (inauthentic), and then after reciting the legislated mention of Allaah, he should perform the Sunnah prayer.
Allaah Knows best.