Sunnah.com reference : Book 2, Hadith 33English translation : Book 2, Hadith 0Arabic reference : Book 2, Hadith 178
Ahmad added to the end of the above Hadith, the story of BIlal's statement in the Fajr Adhan (morning prayer call):
as-Salatu khairun minan-nawm - "Prayer is better than sleep".
وَزَادَ أَحْمَدُ فِي آخِرِهِ قِصَّةَ قَوْلِ بِلَالٍ فِي آذَانِ اَلْفَجْرِ: { اَلصَّلَاةُ خَيْرٌ مِنَ اَلنَّوْمِ }   1‏ .‏‏1 ‏- انظر ما قبله.‏