Raising the voice with remembrance when people finished the obligatory prayer was a practice during the time of the Prophet ﷺ. Ibn Abbas said, "I used to know when they had finished the prayer by hearing it."
أن رفع الصوت بالذكر حين ينصرف الناس من المكتوبة كان على عهد النبي ﷺ. وقال ابن عباس: كنت أعلم إذا انصرفوا بذلك إذا سمعته.
Bukhari narrated it in Adhan (841), and Muslim in Masajid (583:122), both through the route of Abdul-Razzaq, who said, Ibn Jurayj informed us, saying, Amr bin Dinar informed me, that Abu Mabad, the freed slave of Ibn Abbas, informed him, that Ibn Abbas informed him, and he mentioned it.Bukhari narrated it in Adhan (842), and Muslim in Masajid (583:120), both from the hadith of Abu Mabad, the freed slave of Ibn Abbas, from Ibn Abbas, who said, "We used to recognize the completion of the prayer of the Messenger of Allah ﷺ by the takbir."In this is the recommendation of raising one's voice in remembrance following the obligatory prayers. A group of the scholars of hadith held this view. His statement, "by the takbir," refers to remembrance in general, as in the first narration, because the supplications transmitted after the obligatory prayers do not contain the takbir. It is also possible that the Prophet ﷺ raised his voice with the takbir once or twice, so Ibn Abbas heard it, though that was not his usual practice ﷺ.