I would know that the Messenger of Allah ﷺ had finished his prayer by the takbir.
أخبرنا بشر بن خالد العسكري قال حدثنا يحيى بن آدم عن سفيان بن عيينة عن عمرو بن دينار عن أبي معبد
إنما كنت أعلم انقضاء صلاة رسول الله ﷺ بالتكبير
Bukhari transmitted it, in "Adhan," chapter "Remembrance After Prayer," no. 842, and Muslim, in "Mosques," chapter "Remembrance After Prayer," no. 121/583, from the hadith of Sufyan bin Uyayna through this same chain, and it is also in al-Kubra, no. 1258.