How is the Takbir performed before congregational prayer, and are the words of the the Adhan recited once or twice?

The Prophet (peace be upon him) used various authenticated forms of the Adhan (call to prayer). It is sunnah (recommended) to practice all of them to revive the Sunnah and to avoid disputes. Sheikh Ibn 'Uthaymeen mentioned that all forms of the Adhan that have been transmitted through the Sunnah are permissible, and one should perform the Adhan using one form at times and another at other times.

According to the Maliki school, the Adhan consists of seventeen phrases, with the Takbir (Allahu Akbar) recited twice, along with tarji' (repeating the Shahada softly before proclaiming it loudly).

According to the Shafi'i school, it consists of nineteen phrases, with the Takbir recited four times, along with tarji'. Their evidence is the hadith of Abu Mahdhurah in Sahih Muslim.

According to Imam Ahmad and Abu Hanifa, it consists of fifteen phrases, which is the Adhan of Bilal. Their evidence is the hadith of Abdullah ibn Zayd.

Shaykh al-Islam Ibn Taymiyyah was of the view that the correct approach is to permit all forms that have been authenticated from the Prophet (peace be upon him). He considered the variation in the forms of the Adhan to be similar to the variations in Qur'anic recitations and the Tashahhud (testimony of faith). He believed one should perform one form at times and another at other times, because abandoning what is mentioned in the Sunnah might turn the Sunnah into an innovation and lead to disunity.