We used to know that Allah's Messenger (ﷺ) had finished his prayer when we heard the takbir (Allah-O-Akbar).
Sahih Muslim Book 5, Hadith 154
← Previous Hadith I saw the Messenger of Allah (may peace be open him) pronouncing taslim on his right and on his left till I saw the whiteness of his cheek.

Sahih Muslim Book 5, Hadith 153 Next Hadith → We knew the finishing of the prayer of the Messenger of Allah (ﷺ) through takbir. 'Amr (b. Dinar) said: I made a mention of it to Abu Mas'ud. hue he rejected it and said: I never narrated it to you. 'Amr said: He did narrate it before this.

Sahih Muslim Book 5, Hadith 155