Musannaf Ibn Abi Shaybah 6571 — A man who enters the mosque at Fajr

Harithah bin Mudarrib said: Sayyiduna Ibn Mas'ud and Sayyiduna Abu Musa (may Allah be pleased with them both) left the company of Sa'id bin al-As (may Allah be pleased with him), and just then the prayer was established. Sayyiduna Ibn Mas'ud prayed two rak'ahs and then joined the congregation, while Sayyiduna Abu Musa went straight into the row.