It is narrated from Sayyiduna Sa'd al-Qaraz (may Allah be pleased with him) that whenever the Prophet (peace and blessings be upon him) came to Quba', Sayyiduna Bilal (may Allah be pleased with him) would call the adhan, so that the people would know that the Messenger of Allah (peace and blessings be upon him) had arrived and would gather to him. One day he came while Sayyiduna Bilal (may Allah be pleased with him) was not with him, and the Zanj (black) servants began looking at one another. At that, Sa'd (may Allah be pleased with him) climbed up to the place of the adhan. The Messenger of Allah (peace and blessings be upon him) said to him: "O Sa'd, what made you call the adhan?" He said: May my father and mother be your ransom! I saw you with few people, and I did not see Sayyiduna Bilal (may Allah be pleased with him) with you, and I saw these Zanj looking at one another and looking at you, so I feared for you from them, and therefore I called the adhan. He said: "You did right, O Sa'd. Whenever you do not see Bilal with me, call the adhan." So Sa'd (may Allah be pleased with him) called the adhan three times during the lifetime of the Prophet (peace and blessings be upon him).
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5433