It is narrated from Sayyiduna Jubayr bin Mut'im (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was on a journey, and he (peace and blessings be upon him) said: "Who will keep watch over us tonight, so that we do not sleep through the Fajr prayer?" Sayyiduna Bilal (may Allah be pleased with him) said: "I will." So he sat facing the direction of the sunrise, and (sleep) struck their ears, until the heat of the sun woke them. Then they all stood up and led their mounts on, then they performed ablution, Sayyiduna Bilal (may Allah be pleased with him) called the adhan, then they prayed the two rak'ahs of Fajr, then they prayed the Fajr prayer.
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1552