I said to Saeed bin Musayab, "It has reached me that Jabir bin Abdullah used to say they numbered fourteen hundred."Saeed replied, "Jabir told me they were fifteen hundred who pledged allegiance to the Prophet ﷺ."
قلت لسعيد بن المسيب: بلغني أن جابر بن عبد الله كان يقول: كانوا أربع عشرة مائة، فقال لي سعيد: حدثني جابر كانوا خمس عشرة مائة الذين بايعوا النبي ﷺ.
Bukhari narrated it in the chapter on military expeditions (4153), from Salit bin Muhammad, who said, Yazid bin Zuray narrated to us, from Saeed, from Qatada, who said, I said to Saeed, and then he mentioned it.