Salman came to the Messenger of Allah ﷺ when he arrived in Madinah. The Prophet ﷺ said to his companions, "Uncover my back." Salman looked at the seal on the back of the Messenger of Allah ﷺ and believed in him.
جاء سلمان إلى رسول الله ﷺ حين قدم المدينة، فقال النبي ﷺ لأصحابه: ابسطوا. فنظر سلمان إلى الخاتم الذي على ظهر رسول الله ﷺ فآمن به.
Narrated by Imam Ahmad (22997), Bazzar - Kashf Astar (2726), and Tabarani in Al-Kabir (6070), all through the route of Zayd bin Hubab, who said, Husayn bin Waqid told me, Abdullah bin Burayda told me, he said, I heard Burayda, and he related the story of Salman's conversion to Islam at length, which will come in its proper place. It was also narrated by Tirmidhi in Shamail (20), and Hakim (16/2), both through this same route. Hakim said, "Sahih according to the standard of Muslim." I say, its chain is hasan because of Husayn bin Waqid Marwazi, for he is "truthful." And in Taqrib, "Trustworthy, with occasional lapses."As for the story of Salman's conversion to Islam, it has been narrated through several chains, some more sahih than others, and it will come in its proper place. Among them is what Imam Ahmad narrated (23737) from Yaqub, who said, my father told us, from Ibn Ishaq, who said, Asim bin Umar bin Qatada Ansari told me, from Mahmud bin Labeed, from Abdullah bin Abbas, who said, Salman Farsi told me his account from his own mouth. It is a long hadith, in which it is stated, Salman said, "Then I came to the Messenger of Allah ﷺ while he was at Baqi Gharqad. He had followed the funeral procession of one of his companions, wearing two cloaks, and was sitting among his companions. I greeted him, then circled around to look at his back, to see if I could find the seal that my companion had described to me. When the Messenger of Allah ﷺ saw me circling him, he realized I was trying to confirm something that had been described to me. So he threw his cloak off his back, and I looked at the seal and recognized it, and I fell upon it, kissing it and weeping. The Messenger of Allah said to me, 'Turn around.' So I turned, and I related my story to him just as I have related it to you, Ibn Abbas." He said, "The Messenger of Allah ﷺ was pleased for his companions to hear that... ..." Its chain is hasan, because Muhammad bin Ishaq's hadith is hasan when he explicitly states that he heard directly from his source.