Nafi' narrated that 'Abdullah bin 'Umar used to say:
"When the Muslims arrived in al-Madinah they used to gather and try to figure out the time for prayer, and no one gave the call to prayer. One day they spoke about that; some of them said: 'Let us use a bell like the Christians do;' others said, 'No, a horn like the Jews have.' 'Umar, may ,Allah be pleased with him, said: 'Why don't you send a man to announce the time of prayer?' The Messenger of Allah (sallallaahu alayhi wa sallam) said: 'O Bilal, get up and give the call to prayer.'"
Based on the above hadiith, the Jurists agree that it is makruuh for the one who gives the call to prayer to give the adhaan while he is sitting.
Abu Haamid from the Hanaabilah said that if a person gives the Adhaan while he is sitting, the adhaan is baatil (void).
Kuwaiti Encyclopedia of Fiqh
Volume 15 Page 264