If someone enters the mosque during Fajr congregation, should he pray sunnah or join the congregation?

Hazrat Sha'bi narrates that Hazrat Masruq entered the mosque during the Fajr congregation; he had not performed the Sunnah prayers. So, he first offered the Sunnah prayers in a corner, then joined the congregation.

Hasan used to say that a person should first perform the Sunnah prayers of Fajr and then join the congregation.

Hazrat Qasim bin Abi Ayyub narrates that Hazrat Saeed bin Jubair came to the mosque while the Imam was leading the Fajr prayer. Before entering the mosque, he performed the Sunnah of Fajr at the door of the mosque.

Hazrat Abu Usman narrates that Hazrat Umar was leading the prayer, when a man came, performed the Sunnah of Fajr in a corner of the mosque, and then joined the congregation.

Hazrat Haritha bin Mudarrib narrates that Hazrat Ibn Mas’ud and Hazrat Abu Musa got up from the company of Sa’id bin ‘As. Meanwhile, the prayer was established. Hazrat Ibn Mas’ud performed two rak’ahs and then joined the congregation. Hazrat Abu Musa entered the row from the very beginning.

Hazrat Dawood bin Ibrahim says that I asked Hazrat Tawus, "Can I offer two rak'ahs during the iqamah?" He replied, "Do you have the strength for it?"

Hazrat Ikrimah says that whether you recite (Qur'an) in the Sunnah of Fajr or not, if you do recite, then keep it brief, but definitely perform these two rak'ahs, even if you have to offer them on the way.

Hazrat Mujahid says that if you enter the mosque during the congregation and you have not performed the Sunnah of Fajr, then first perform these Sunnah, even if you fear missing the first rak'ah.

Hazrat Wabra says that I also saw Hazrat Ibn Umar (may Allah be pleased with them both) doing the same. Someone told me that Hazrat Ibn Umar (may Allah be pleased with them both) did this twice: once, when he came during the congregation, he performed the Sunnah prayers of Fajr in a corner of the mosque; then, the second time, he joined the congregation without having performed the Sunnah prayers of Fajr.

6576 - (And) (1) Someone who saw him narrated to me (2) that he did it twice: he came once (while they) (3) were in prayer (so he prayed them) (4) in the side of the mosque, then he entered another time and prayed with them and did not pray them (5).

Ibrahim has considered it disliked to perform the Sunnah prayers of Fajr in the mosque during the congregation. He states that these Sunnah prayers should be performed during the congregation at the corner or at the door of the mosque.

Hazrat Abu Darda says that sometimes I come to the mosque during the congregation of the Fajr prayer; I offer the two Sunnah (units) and then join the congregation.