Musnad Ahmad 30:2283

Qutaybah narrated to us, 'Abd al-'Aziz narrated to us, from Thawr, from Abu al-Ghayth, from Abu Hurayrah, that he said: We were sitting with the Prophet, peace and blessings of Allah be upon him, when Surah al-Jumu'ah was revealed to him. When he recited: {And [to] others of them who have not yet joined them} [al-Jumu'ah: 3], a man said: Who are they, O Messenger of Allah? The Prophet, peace and blessings of Allah be upon him, did not answer him until he asked him once, or twice, or three times. And among us was Salman al-Farisi. He said: So the Prophet, peace and blessings of Allah be upon him, placed his hand upon Salman and said: "If faith were at the Pleiades, men from these people would attain it."