Abdullah bin Abi Musa narrates: Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) came to us (i.e. in the mosque) when the imam had begun leading the Subh prayer. Sayyiduna Abdullah (may Allah be pleased with him) offered two rak'ahs standing by a pillar; he had not yet offered the two rak'ahs of Fajr (i.e. the sunnah).
Al-Tabarani recorded it, and its narrators have been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2392