Sa'id bin al-Musayyib narrates from Sayyiduna Bilal (may Allah be pleased with him); Bilal (may Allah be pleased with him) said: "We were with the Prophet (peace and blessings be upon him) on a journey, and the people slept until the sun rose. So he (peace and blessings be upon him) ordered Bilal, and he gave the adhan. Then they prayed the two (sunnah) rak'ahs of Fajr, then they prayed the (obligatory) Fajr prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1075