Mishkat al-Masabih 11 Business Transactions كتاب البيوع 272

وَعَنِ الْجَارُودِ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «ضَالَّةُ الْمُسْلِمِ حَرَقُ النَّارِ» . رَوَاهُ أَبُو دَاوُد

Al-Jarud reported God’s Messenger as saying, “A Muslim’s lost property is a cause of the flame of hell*.” * i.e. when the finder, wishing to keep the article, does not make public the fact that he has found it. Darimi transmitted it.