Mishkat al-Masabih 11 Business Transactions كتاب البيوع 268

وَعَنْهُ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ آوَى ضَالَّةً فَهُوَ ضَالٌّ مَا لم يعرفهَا» . رَوَاهُ مُسلم

He reported God’s Messenger as saying, “He who gives refuge to a stray is astray himself as long as he does not make the matter known.” Muslim transmitted it.