To the man whom he taught how to pray, he said:
"When you intend to pray, perfect your ablution, then face the qiblah, then pronounce the takbīr, and then recite."
The clear implication is that only the first takbīr is obligatory; had the other takbīrs been compulsory, he would have mentioned them just as he mentioned the other obligatory parts of the prayer.
As for what has been transmitted of his practice, it includes the ḥadith of Abū Hurayrah:
"He would pray and utter the takbīr whenever he bowed or rose, and would then say, ‘Among you I am the one whose prayer most resembles the prayer of the Messenger of Allah – peace and blessings be upon him.’"
Another is the report of Muṭarrif ibn ʿAbd Allāh ibn al-Shikhkhīr, who said:
"I and ʿImrān ibn Ḥuṣayn prayed behind ʿAlī ibn Abī Ṭālib – may Allah be pleased with him. Whenever he prostrated he said the takbīr, and whenever he raised his head from bowing he said the takbīr. When the prayer was over and we had left, ʿImrān took my hand and said, ‘This man has reminded me of the prayer of Muhammad – peace and blessings be upon him.’"
Those who judge every takbīr to be obligatory hold fast to these transmitted actions, arguing that every act of the Prophet that clarifies a duty is itself mandatory, as he said: “Pray as you have seen me pray,” and, “Take your rites from me.”
The first group responds that these reports only show that the Companions were concerned with completing the takbīr; hence Abū Hurayrah’s words, “Among you I most resemble the prayer of the Messenger of Allah,” and ʿImrān’s statement, “This reminded me of the prayer of Muhammad – peace and blessings be upon him.”
As for the view that treats all the takbīrs as merely supererogatory, it is weak; its advocate seems to have compared them to the other remembrances within the prayer that are not obligatory, and to have likened the opening takbīr to the rest.
Abū ʿUmar Ibn ʿAbd al-Barr said:
“Among the evidences supporting the majority’s view is what Shuʿbah ibn al-Ḥajjāj narrated from al-Ḥasan ibn ʿImrān, from ʿAbd Allāh ibn ʿAbd al-Raḥmān ibn Abzā, from his father, who said: ‘I prayed with the Prophet – peace and blessings be upon him – and he did not complete the takbīr; and I prayed with ʿUmar ibn ʿAbd al-ʿAzīz, and he too did not complete the takbīr.’”