The Messenger of Allah ﷺ said, "A lost item belonging to a Muslim is a flame of the Fire."
قال رسول الله ﷺ: ضالة المسلم حرق النار.
Ibn Majah (2502), Ahmad (16314), Ibn Hibban (4888), Bayhaqi (1916), and Baghawi in his commentary (2209) all narrated it from the hadith of Yahya bin Saeed, from Humayd Tawil, from Hasan, from Mutarrif bin Abdullah bin Shikhir, from his father, who related it. Its chain of transmission is sahih. Hasan here is Hasan Basri, the well-known imam, who was known to practice tadlis, but the fact that Ibn Hibban included it in his Sahih suggests that he did not practice tadlis in this instance. Qatada also followed him in narrating it from Mutarrif in the same manner. Abu Naeem narrated it in al-Hilyah (9/33).As for his saying "the burning of the fire," it means the cause of his entering the fire if he took possession of it and did not make it known.