Musannaf Ibn Abi Shaybah 6602 — The two rakahs of Fajr when one misses

Once Sayyiduna Ibn Umar (may Allah be pleased with them both) came at the time of the Fajr prayer, and the people were praying. He had not yet prayed the sunnah of Fajr. He joined the congregation, then remained sitting in his place of prayer, and at the time of duha (forenoon) he made up those rak'ahs.