Most African muslim places White cloth in the mosque and sits around it to worship and mentioning the numerous name of Allah after magrib prayer is been done. My question is: Is it a Sunna?
All perfect praise be to Allah
While it is still unclear what they do, but it seems like a practice(of a group) in our country where after each prayer, people loudly repeat KALIMA and than darood in the end, which is a biddah. So, if they cannot provide any supporting evidence for what they are doing. It is in the same category.
There is a whole list of SUNNAH DUA(sayings) after prayer which we should say and they are enough for us. I don't know why people have started prefering THEIR zikr on sunnah zikr.
EDIT: I thought i should include a hadith which covers a similar happening in masjid al-nabvi. Hope it will be helpful.
al-Darimi in the Muqaddima of his Sunan narrated from al-Hakam ibn al-Mubarak who narrates from Amr ibn Salima al-Hamadani:
We used to sit by the door of Abd Allah ibn Masud before the Morning Prayer, so that when he came out we would walk with him to the mosque. (One day) Abu Musa al-Ashari came to us and said: "Has Abu Abd al-Rahman come out yet?" We replied No. So he sat down with us until he came out. When he came out, we all stood along with him, so Abu Musa said to him: "O Abu Abd al-Rahman! I have just seen something in the mosque which I considered wrong, but all praise is for Allah, I did not see anything except good in it." He inquired: "What is it?" Abu Musa replied: "If you live you will see it. I saw in the mosque people sitting in circles awaiting the Prayer. In each circle they had pebbles in their hands and a man would say: Repeat allahu akbar a hundred times. So they would repeat it a hundred times. Then he would say: say la ilaha illallah a hundred times. So they would say it a hundred times. Then he would say: say subhanallah a hundred times. So they would say it a hundred times." Ibn Masud asked: "What did you say to them?" Abu Musa said: "I did not say anything to them. Instead I waited to hear your view on it." Ibn Masud replied: "Would that you had ordered them to count their evil deeds and assured them that their good deeds would not be lost!" Then we went along with him until he came to one of these circles whereby he stood and said: "What is this I see you doing?" They replied: "O Abu Abd al-Rahman! These are pebbles upon which we are counting takbir, tahlil and tasbih." He said: "Count your evil deeds instead. I assure you that none of your good deeds will be lost. Woe to you, O Umma of Muhammad, sallallahu alayhi wa sallam, how quickly you go to destruction! Here are your Prophet's Companions available in abundance (mutawafirun). And there are his clothes which have not yet decayed and his bowl which is unbroken. By Him in Whose Hand is my soul! Either you are following a Religion that is better guided than the Religion of Muhammad sallallahu alayhi wa sallam, or you are opening a door of misguidance." They said: "O Abu Abd al-Rahman! By Allah, we only intend good!" He said: "How many are there who intend good but do not achieve it. Indeed Allah's Messenger said to us: A people will recite the Qur'an but it will not pass beyond their throats. By Allah! I do not know, but perhaps most of them are from among you." Then he left them. Amr ibn Salima said: We saw most of those people fighting against us on the day of Nahrawan, on the side of the Khawarij.
(there are also many other isnaad of this hadees)
Now i should also have to mention that their is a group of people who does not agree to this defination of biddah which is used in this hadees. for details on defination of biddah you can refer to this question and tits answers.