It is confirmed that the Prophet (peace be upon him) led a two rak’ahs prayer with the prophets who were gathered by Allah in Masjid Al-Aqsa in the night of Isra. And it wasn’t specified in the trusted narratives if this prayer was a Fard or a Nafila, and it’s needless to go through this; because it is not of practical benefit. Though, it’s undisputed that the five prayers of Fard were obliged in the night of Isra and Miraj after the Prophet’s ascendance to the heavens and his conversation with Allah. And the prayer with the prophets that he led was before the Miraj.
Source: Permanent Committee for Islamic Research and Issuing Fatwas (paraphrased)