On the authority of Abu Harayrah (may Allah be pleased with him) from the Prophet (PBUH), who said:

Allah (glorified and exalted be He) has supernumerary angels who rove about seeking out gatherings in which Allah's name is being invoked: they sit with them and fold their wings round each other, fillin that which is between them and between the lowest heaven. When [the people in the gathering] depart, [the angels] ascend and rise up to heaven. He (the Prophet p.b.u.h.) said: Then Allah (mighty and sublime be He) asks them - [though] He is most knowing about them: From where have you come? And they say: We have come from some servants of Yours on Earth: they were glorifying You (Subhana llah), exalting you (Allahu akbar), witnessing that there is no god but You (La ilaha illa llah), praising You (Al-Hamdu lillah), and asking [favours] of You. He says: And what do they ask of Me? They say: They ask of You Your Paradise. He says: And have they seen My Paradise? They say: No, O Lord. He says: And how would it be were they to have seen My Paradise! They say: And they ask protection of You. He says: From what do they ask protection of Me? They say: From Your Hell-fire, O Lord. He says: And have they seen My Hell-fire? They say: NO. He says: And how would it be were they to have seen My Hell-fire: They say: And they ask for Your forgiveness. He (the Prophet p.b.u.h) said: Then He says: I have forgiven them and I have bestowed upon them what they have asked for,and I have granted them sanctuary from that from which they asked protection. He (the Prophet p.b.u.h) said: They say: O Lord, among then is So-and-so, a much sinning servant, who was merely passing by and sat down with them. He (the Prophet p.b.u.h) said: And He says: And to him [too] I have given forgiveness: he who sits with such people shall not suffer.

It was related by Muslim (also by al-Bukhari, at-Tirmidhi, and an-Nasa'i).