All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
There is no doubt that it is permissible to recite the Athkaar (pl. of Thikr [expressions of remembrance of Allah]) after the athaan and after concluding the prayer by saying the Salaam, and it is acceptable for the imaam to raise his voice with some Athkaar after the prayer, as a way of teaching the people, but doing so with a collective voice is not permissible.
Therefore, the Athkaar that are said by the imaam and the worshipers collectively with some additions that were not set by the Shariah are an innovation in the religion, even if it is with the intention of remembering Allah, because the Athkaar are part of the acts of worship, which must fulfill two conditions: the condition of sincerity, and the condition of being in conformity with the guidance of Prophet . Sincerity alone is not enough.
There is no doubt that remembering Allah is something required and permissible, but it should not be done in an innovated manner. Had the mention of Allah been permissible in whichever manner, then Ibn Mas‘ood would not have objected to those who gathered in the mosque, making Tasbeeh and Tahleel (saying La Ilaaha illa Allah) in a manner that was not taught by the Prophet and his Companions.
Ad-Daarimi narrated with an authentic chain of narrators:
"Ibn Mas‘ood saw groups of people sitting in circles in the mosque waiting for the prayer. Inside each circle, there was a man, and they had pebbles in their hands. He would say, 'Say Allahu Akbar' (Allah is the Most Great) one hundred times, and they would say 'Allahu Akbar' one hundred times. He would say, 'Say Laa ilaha illa Allah' (there is no god worthy of worship but Allah) one hundred times, and they would say 'Laa ilaha illa Allah' one hundred times. He would say, 'Say Subhan Allah' (Glory be to Allah) one hundred times, and they would say 'Subhan Allah' one hundred times. So these people were gathered to mention Allah, and Ibn Mas‘ood came to one of these circles, and he stood over them and said, 'What is this that I see you doing?' They said, 'O Abu ‘Abdur-Rahmaan, these are pebbles with which we count the Takbeer, Tahleel, and Tasbeeh.' He said, 'Rather, count your bad deeds, for I guarantee to you that none of your good deeds will be lost. Woe to you, O Ummah (community) of Muhammad! How quickly you have become doomed! His Companions are still alive and his clothes are not yet worn out, and his vessel is not yet broken. By the One in Whose Hand my soul is, you are either following a way that is more guided than the way of Muhammad or you are opening the door to misguidance.' They said, 'By Allah, O Abu ‘Abdur-Rahmaan, we intended nothing but good…'” [Abridged]
Ibn Mas‘ood did not find them drinking alcohol; rather, he found them mentioning Allah Almighty, but because their method in mentioning Allah was an innovated method, he denied the way they were doing it collectively and their counting on pebbles.
Shaykh Abdul Mohsin al-‘Abbaad said in his commentary on Sunan Abi Daawood "This narration is authentic from ‘Abdullah ibn Mas‘ood, who denied them doing this act, which was that they were praising Allah collectively by using pebbles, and all this is contrary to the Sunnah... He denied them not conforming to the Sunnah in the way they did it (collectively) and the manner in which they performed it (using pebbles)."
Therefore, that imaam and those worshipers must fear Allah and do the mention after the obligatory prayers in the same manner as the Prophet and his Companions did it, without any addition or decrease.
Also, it was not reported that the Prophet used to recite the last verses of Soorah Al-Hashr after the Fajr and the Maghrib prayers, and he did not recite the last verses of Soorah Al-Baqarah after the ‘Ishaa’ prayer, and they were not used to recite Al-Faatihah (after the prayers). Indeed, ‘Aa’ishah narrated that the Prophet said, "Whoever introduces into this matter of ours (Islam) that which is not part of it, it will be rejected." [Al-Bukhaari and Muslim]
In the wording by Muslim, it reads, "Whoever performs an act which is not in accordance with ours [our religion], it will be rejected." [Muslim]
Dear brother, if you lead them in prayer, do not do those innovations.
Allah knows best.