Is saying 'Allahu Akbar' (the Takbir) after obligatory prayer mandatory?
Chapter on the Two Eid Prayers
Al-Mughni
Book of Friday Prayer
Primary text
Saying 'Allahu Akbar' upon facing the Qibla after prayer is established. Ahmad narrated this from Ibrahim, and Abu Bakr affirmed that this is the ruling acted upon because this remembrance is specific to the prayer, similar to the Adhan and Iqamah. If one forgets the Takbir until exiting the mosque, one does not say it. This position is held by the Companions of the Ra'y school because it is specific to the subsequent prayer, thus resembling the prostration of forgetfulness (sujud al-sahw). If one remembers it while still inside the mosque, one should return to one's place, sit, face the Qibla, and then say the Takbir.
Supporting text
It is permissible to say the Takbir in any manner desired, based on the narration of Jabir that the Prophet (peace be upon him) addressed them saying, 'Allahu Akbar, Allahu Akbar.' Alternatively, it is permissible to say the Takbir later, even after exiting, similar to supplication and other prescribed remembrance after prayer, as it is a general remembrance and purification (Taharah) is not stipulated for it.