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 the Book of Adhan (841), and Muslim in the Book of Mosques (583:121), both of them through Abdul-Razzaq, who said, Ibn Jurayj informed us, he said, Amr bin Dinar informed me, that Abu Mabad, the freed slave of Ibn Abbas, informed him, that Ibn Abbas informed him, he said (then he mentioned it).This contains evidence for those scholars who held it recommended to raise the voice with takbir and remembrance after the prescribed prayer, among them Ibn Hazm the Zahiri.As for Shafii, he held that raising the voice with takbir and remembrance was not recommended. He interpreted Ibn Abbas's statement to mean that the Prophet ﷺ raised his voice with it for a brief period so that people would learn the manner of the remembrance, not that he used to raise his voice with it all the time. See Majmu Sharh Muhazzab (3/487).As for his statement, "I would know when they had finished," it apparently indicates that he did not attend the congregational prayer at some times because of his young age. This is what Qadi Iyad inferred, as mentioned in Fath al-Bari (2/326).