Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 532
Narated By Abdullah ibn Umar : Bilal made a call to prayer before the break of dawn; the Prophet (pbuh), therefore, commanded him to return and make a call: Lo! the servant of Allah (i.e. I) had slept (hence this mistake).
The version of Musa has the addition: He returned and made a call: Lo! the servant of Allah had slept.