768 - Muhammad ibn Abi Hazm narrated to us, he said: Muhammad ibn Bakr narrated to us, he said: Hisham ibn Hassan informed us, from al-Hasan—regarding the one who forgets to open his prayer with the Takbir—he said: "The Takbir of bowing suffices him. If he comes while they are in bowing, he should say two Takbirs: one Takbir with which he enters into the prayer, and one Takbir for his bowing. If he forgets, one suffices him."
And I heard Ishaq—another time—say: "As for those of the Kufans who say: 'He says one Takbir for both the opening and the bowing,' it is a mistake; because it is not permissible for the obligatory (fard) to be mixed with a Sunnah or voluntary act, and the opening Takbir is obligatory; through it one enters the state of consecration (tahrrum)."
769 - Yahya ibn Abd al-Hamid narrated to us, he said: Sharik narrated to us, from his father, from Mujahid; and Mughirah, from Ibrahim—regarding the man who comes to the Imam while he is bowing—he said: "He says two Takbirs; if he says one, it suffices him."
770 - Abbas ibn Abd al-Azim narrated to us, he said: Abd al-Rahman ibn Mahdi and Abu Dawud narrated to us, from Ibrahim ibn Sa'd, from al-Zuhri, that Zayd ibn Thabit and Abdullah ibn Umar used to, when they arrived to the Imam while he was bowing, say one Takbir, and they would bow with that (same) Takbir.
771 - Abbas narrated to us, he said: I heard Abd al-Rahman ibn Mahdi say: "He says two Takbirs; and if he says one Takbir intending with it both the opening and the bowing, it suffices him."
(1) This is as it appears in the original, and the correct form is "qala" (they two said), referring to Mujahid and Ibrahim.