Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.107.

To “sing” the azhan or to state it in improper Arabic by adding a letter or lengthening the sound of a vowel, and so on, is disliked. If it changes or obscures the meaning of what is said, it becomes forbidden.

Reported Yahya al-Baka’, “I saw Ibn ‘Umar say to a man, ‘I am mad at you for the sake of Allah.’ Then he said to his companions, ‘He sings in making his azhan, and he takes wages for it.”‘