It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that when the Muslims came to Madinah, then he narrated the hadith about the beginning of the adhan; at its end it says that the Messenger of Allah (peace and blessings be upon him) said: "O Bilal, stand and call the adhan for the prayer!"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1839
Abd al-Jabbar bin Wa'il narrates from his father (may Allah be pleased with him): "It is a right and the Sunnah that a man should give the adhan standing, while in a state of purity."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1840