Here are some famous ahadiths regarding dhikr circles.

Hadith Qudsi, "Those that remember Me in their heart, I remember them in My heart; and those that remember Me in a gathering (i.e. that make mention of Me), I remember them (i.e. make mention of them) in a gathering better than theirs.

The above hadith constitutes proof that dhikr done outloud in a gathering is an exalted kind of dhikr which is mentioned at the Highest Assembly (al-mala' al-a`la).

Jabir reported that the Prophet (Peace be upon him), said: "The best remembrance of Allah is to repeat la ilaha illallah and the best prayer (du'a) is al-hamdu lillah (all praise belongs to Allah)." (Narrated by Nasa'i, Ibn Majah, and Hakim who declared its chain sound).

In Bukhari and Muslim: The Prophet (Peace be upon him) said that Allah has angels roaming the roads to find the people of dhikr, i.e. those who say La Ilaha Illallah and similar expressions, and then they find a group of people (qawm) reciting dhikr, they call each other and encompass them in layers until the first heaven -- the location of which is in Allah's knowledge. (This is to say an unlimited number of angels are going to be over that group).

This is further indicated by the Hadith Qudsi which runs: "Allah says: I am to my servant as he expects of Me, I am with him when he remembers Me. If he remembers Me in his heart, I remember him to Myself, and if he remembers me in an assembly, I mention him in an assembly better than his..." (Bukhari and Muslim) Thus, silent dhikr is differentiated fron dhikr said outloud by His saying: "remembers Me within himself," meaning: "silently," and "in an assembly," meaning "aloud."

The Holy Prophet (Peace be upon him) said: 'hatta yaqulu majnun' "Remember Allah as much as you want, until people say that you are crazy and foolish" (Narrated by Ahmad in his Musnad, Ibn Hibban in his Sahih, and al-Hakim who declared it sahih); that is: do not care about them!

Ibn `Umar reported that the Prophet (Peace be upon him) said: "When you pass by the gardens of Paradise, avail yourselves of them." The Companions asked: "What are the gardens of Paradise, O Messenger of Allah?" He replied: "The circles of dhikr. There are roaming angels of Allah who go about looking for the circles of dhikr, and when they find them they surround them closely." Tirmidhi narrated it (hasan gharib) and Ahmad.

Abu Sa`id Al-Khudri and Abu Huraira (RA) reported that the Prophet (Peace be upon him) said, "When any group of men remember Allah, angels surround them and mercy covers them, tranquillity descends upon them, and Allah mentions them to those who are with Him." Narrated by Muslim, Tirmidhi, Ahmad, Ibn Majah, and Bayhaqi.

Muslim, Ahmad, and Tirmidhi narrate from Mu`awiya that the Prophet (Peace be upon him) went out to a circle of his Companions and asked: "What made you sit here?" They said: "We are sitting here in order to remember / mention Allah (nadhkurullah) and to glorify Him (wa nahmaduhu) because He guided us to the path of Islam and he conferred favours upon us." Thereupon he adjured them by Allah and asked if that was the only purpose of their sitting there. They said: "By Allah, we are sitting here for this purpose only." At this the Prophet said: "I am not asking you to take an oath because of any misapprehension against you, but only because Gabriel came to me and informed me that Allah, the Exalted and Glorious, was telling the angels that He is proud of you!" Note that the hadith stated jalasna -- we sat -- in the plural, not singular. It referred to an association of people in a group, not one person.

The Prophet, peace be upon him, would often tell his Companions, "Shall I tell you about the best of deeds, the most pure in the sight of your Lord, about the one that is of the highest order and is far better for you than spending gold and silver, even better for you than meeting your enemies in the battlefield where you strike at their necks and they at yours?" The Companions replied, "Yes, O Messenger of Allah!" The Prophet, peace be upon him, said, "Remembrance of Allah." (Narrated by Tirmidhi, Ahmad, and Hakim who declared its chain of narrators sound

To quote a few Hadith about Dhikr, the Holy Prophet ( Sallallahu Alaihi Wasallam) says: " There is a polish for everything, for the hearts it is Dhikr of Allah."

He has also said: " Those who remember Allah and those who do not are like those who are alive and those who are dead." (i.e. who remember Allah and celebrate, His praises are alive and those who do not are dead.)