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.