Musannaf Ibn Abi Shaybah 6576 — A man who enters the mosque at Fajr

(Wabarah said:) Someone who saw him told me that Sayyiduna Ibn Umar (may Allah be pleased with them both) did it in two ways: once he came while they were in the congregational prayer, and he prayed the sunnah rak'ahs of Fajr in a corner of the mosque; and another time he came and joined the congregation without having prayed the sunnah rak'ahs of Fajr.