“Abū Hurayra, may Allah be pleased with him, used to pronounce the takbīr in prayer whenever he rose or lowered himself. I said, ‘Abū Hurayra, what is this takbīr?’ He replied, ‘This is indeed the prayer of the Messenger of Allah, may Allah bless him and grant him peace.’”

All of this refers to the imam pronouncing the takbīr aloud. This is why they called it “completing the takbīrs”: completing them involved raising the voice and pronouncing them at every downward and upward movement.

This is made clear by what al-Bukhārī mentions in the “Chapter on Pronouncing the Takbīr When Rising after Two Rakʿahs.” He says, “Ibn al-Zubayr used to pronounce the takbīr as he rose.” Al-Bukhārī then relates through Fulayh ibn Sulaymān, from Saʿīd ibn al-Hārith, who said:

“Abū Saʿīd led us in prayer and pronounced the takbīr aloud when he raised his head from prostration, when he prostrated, when he rose, and when he stood up after two rakʿahs. He said, ‘This is how I saw the Messenger of Allah, may Allah bless him and grant him peace.’”

Al-Bukhārī then follows this with the hadith of Mutarrif, who said:

“Imrān ibn Husayn and I prayed behind Alī ibn Abī Tālib, may Allah be pleased with him. Whenever he prostrated, he pronounced the takbīr; whenever he rose, he pronounced the takbīr; and whenever he stood up after two rakʿahs, he pronounced the takbīr. When he concluded the prayer with the salām, Imrān ibn Husayn took my hand and said, ‘This man has indeed led us in the prayer of Muhammad, may Allah bless him and grant him peace,’ or he said, ‘This man has indeed reminded me of the prayer of Muhammad, may Allah bless him and grant him peace.’”

This makes clear that the discussion concerns only pronouncing the takbīr aloud. As for the practice of pronouncing the takbīr itself, no one was unaware of it. Nor, moreover, is it unclear