Is it true that the supplication for completing the Qur'an in prayer is not established?

Question:
He says: Honorable Shaykh, may Allah grant you success, is it true that the supplication for completing the Qur'an in prayer is not established? And if so, what should I do if I am a ma'mum (one praying behind the imam) and the imam recites it? I follow the imam and say "Ameen" to his supplication. Imam Ahmad (may Allah have mercy on him) used to do this, as mentioned by the author of al-Mughni.

Answer:
He used to attend the completion of the Qur'an; he used to attend the supplication for completing the Qur'an in Ramadan behind the imam and say "Ameen" to his supplication. The supplication is established; the supplication is established in prayer. It is reported that the Prophet (peace and blessings be upon him) would supplicate in prayer, and that when he passed a verse of mercy, he would stop and ask, and when he passed a verse of warning, he would stop and seek refuge in Allah (Mighty and Majestic). So supplication is established in prayer, and among it is the supplication for completion.