Abdullah bin Umar (may Allah be pleased with them both) narrates: Once, Bilal (may Allah be pleased with him) gave the adhan before true dawn, so the Prophet Muhammad (peace and blessings be upon him) instructed him to go back and announce three times: "Be aware! The servant had fallen asleep (meaning he made a mistake)." He went back and loudly said: "Be aware! The servant had fallen asleep." He said these words three times. Another narrator has corroborated this, however, this narrator is weak.