Question: What is the evidence that Takbir is said after completing the prayers?
Answer: The Hadith of Ibn ‘Abbas radiallahu anhummah:
ما كنا نعرف انتهاء صلاة النبي صلى الله عليه وآله وسلم إلا بالتكبير
We used to know that Allah’s Messenger (ﷺ) had finished his prayer when we heard the Takbir
[Reported by al-Bukhari no. 842 and Muslim no. 583]
What is more deserving is that it be said with voice that he can hear himself and not disturb the others by raising his voice. His statement in the Hadith:
ما كنا نعرف
We used to know
Can be interpreted to mean that he used to raise his voice in terms that he heard himself and those around him heard him. This is due to the Hadith:
لا يجهر بعضكم على بعض بالقرآن فكلكم يناجي ربه
Do not raise your voices one over the other with the Quran, for each of you is speaking with his Lord [ie. in Prayer]
[Reported by Ahmad 3/94, Abu Dawood 4/123, Abd ibn Humayd in al-Muntakhab 2/66, and it is in Saheeh ul-Musnad no. 419]
[Taken from: https://www.sh-yahia.net/show_books_36.html ]
Translated by
Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan