We were sitting with the Prophet ﷺ when Surah al-Jumuah was revealed to him. When he recited "And to others among them who have not yet joined them" [Jumuah 62:3], a man asked, "Who are they, Messenger of Allah?"The Prophet ﷺ did not respond to him until he had asked once, twice, or three times. Salman Farsi was among us. The Prophet ﷺ placed his hand on Salman and said, "If faith were at the Pleiades, some men from these people would reach it."
كنا جلوسا عند النبي ﷺ إذ نزلت عليه سورة الجمعة، فلما قرأ: {وءاخرين منهم لما يلحقوا بهم} قال رجل: من هؤلاء يا رسول الله؟ فلم يراجعه النبي ﷺ حتى سأله مرة أو مرتين أو ثلاثا. قال: وفينا سلمان الفارسي. قال: فوضع النبي ﷺ يده على سلمان، ثم قال: لو كان الإيمان عند الثريا لناله رجال من هؤلاء.
Bukhari narrated it in tafsir (4898) and Muslim in the merits of the companions (2546:231), both through Abdul-Aziz bin Muhammad, who said, Thawr told me, from Abu Ghayth, from Abu Hurairah, who said, and then mentioned it. The wording given is Muslim's, and Bukhari's wording is similar.