Sayyiduna al-Sa'ib bin Yazid (may Allah be pleased with him) says that on Friday, in the time of the Prophet (peace and blessings be upon him), Sayyiduna Abu Bakr and Sayyiduna Umar (may Allah be pleased with them), the first adhan was given when the imam sat on the minbar. When the caliphate of Sayyiduna Uthman (may Allah be pleased with him) came and the number of people increased, he ordered the third adhan, which was given at the place called al-Zawra'; then the practice remained like that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5744
Yunus also mentioned the hadith in the same way, but he says that the first adhan on Friday would be when the imam sat on the minbar, and that it was Sayyiduna Uthman (may Allah be pleased with him) who ordered the third adhan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5745
Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that when the Prophet (peace and blessings be upon him) came out on Friday and sat on the minbar, Bilal (may Allah be pleased with him) would then give the adhan.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5746
(a) Nafi' narrates from Ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) used to deliver two sermons. When he ascended the pulpit, he (peace and blessings be upon him) would sit until the mu'adhdhin had finished the call to prayer, then he (peace and blessings be upon him) would stand and deliver the sermon. Then he would sit and not speak to anyone, then he would stand and deliver the sermon.
(b) Anas bin Malik (may Allah be pleased with him) relates the story of the pulpit, that they made for him (peace and blessings be upon him) a pulpit with two steps, and he (peace and blessings be upon him) would sit on the third. When the Prophet (peace and blessings be upon him) sat on it, the trunk began to weep.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5747