754 - Yahya ibn 'Abd al-Hamid narrated to us, saying: Abu Mu'awiyah narrated to us, from Abu Sufyan Tarif al-Sa'di, from Abu Nadrah, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "The key to prayer is purification (tahur), its sanctity (tahrim) is the Takbir, and its releasing (tahlil) is the Taslim."
Chapter: A man who says the opening Takbir before the Imam
755 - Ahmad ibn Nasr narrated to us, saying: Habban ibn Musa narrated to us, from Sufyan, from 'Abd al-Malik, from 'Abd Allah ibn al-Mubarak, that he did not consider his prayer complete if he finished the first Takbir exactly with the Imam.
756 - Abu Bakr Ahmad ibn Muhammad narrated to us, saying: 'Arim narrated to us, saying: Khalid ibn al-Harith narrated to us, saying: I heard 'Ubayd Allah ibn al-Hasan being asked about the Imam when he says the Takbir; when should those behind him say it? He said: "When he completes the intonation of the Takbir, then say it."
Chapter: Reciting the Takbir aloud behind the Imam
(1) This is how it appears in the original, but the correct reading is "from Sufyan ibn 'Abd al-Malik," who is among the teachers of Habban and the students of Ibn al-Mubarak.