(Muslim and Ibn Mājah) then the Prophet (ṣallá Allāhu ʿalayhi wa-sallam) would begin the prayer by saying: "Allāhu akbar" and he instructed the one who prayed incorrectly to do so, as previously mentioned. And he said to him: (al-Ṭabarānī with a sound isnād) "Indeed, the prayer of any person is not complete until he performs ablution, placing the ablution in its proper places, then says: Allāhu akbar" (Abū Dāwūd and al-Tirmidhī, authenticated by al-Ḥākim and agreed upon by al-Dhahabī). And he would say: "The key to prayer is purification, its prohibition is the takbīr, and its permission is the taslīm" (Aḥmad and al-Ḥākim, authenticated and agreed upon by al-Dhahabī). And (he would raise his voice with the takbīr so that those behind him could hear) (Muslim and al-Nasāʾī). And (when he was ill, Abū Bakr would raise his voice to convey the Prophet's takbīr to the people) (Aḥmad and al-Bayhaqī with a sound isnād). And he would say: "When the imam says: Allāhu akbar, then say: Allāhu akbar."