Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.097.

There are three ways to make the azhan:

1. Make four takbir at the beginning and say the rest of the phrases twice, without any repetition, except for the last statement of la illaha illa-lah. So, the azhan would be made up of fifteen phrases, as in the preceding hadith of ‘Abdullah.

2. Make four takbir and then repeat ashhadu an la ilaha illal-lah, twice, and ashhadu anna Muhammad ar-Rasool-lal-lah twice, in a low voice, then repeat them again in a louder voice. Abu Mahzhura reported that the Prophet, upon whom be peace, taught him an azhan consisting of nineteen phrases. This hadith is related by “the five.” At-Tirmizhi called it hassan sahih.

3. Make two takbir and repeat the “statements of witness,” making the number of phrases seventeen. Muslim records that Abu Mahzhurah related that the Prophet, upon whom be peace, taught him the following azhan: Allahu akbar, Allahu akbar. Ashhadu alla ilaha illal-lah, ashhadu alla ilaha illal-lah. Ashhadu anna Muhammad ar-Rasool-lal-lah, ashhadu anna Muhammad ar-Rasool-lal-lah. Then repeat ashhadu alla ilaha illal-lah (twice), ashhadu anna Muhammad ar-Rasool-lal-lah (twice), hayya ‘alas-salah (twice), hayya ‘alal-falah (twice). Allahu akbar, Allahu akbar. La ilaha illal-lah. “