Reference: http://alfawzan.af.org.sa/node/15650
Question: He says we should say [صدق الله العظيم] after reading the Qur.aan.
Response: There is no evidence for [saying] this; there is no evidence to bind one to [say] [صدق الله العظيم] after reading [the Qur.aan]. Rather, that which has been mentioned to be said when beginning [the recitation] is as Allaah Jalla wa ‘Alaa says:
This is what has been legislated regarding the Qur.aan – when beginning [the recitation] to seek refuge in Allaah from Shaytaan the outcast. As for when ending the recitation, then nothing has been [authentically] mentioned that [صدق الله العظيم] is to be said.