Muwatta Malik, Book 36, Hadith 50 (USC-MSA) ⓘ
Malik related to me from Yahya ibn Said from Said ibn al-Musayyab that Umar ibn al-Khattab said while he was leaning his back against the Kaba, "Whoever takes a stray is astray."
وَحَدَّثَنِي مَالِكٌ، عَنْ يَحْيَى بْنِ سَعِيدٍ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، أَنَّ عُمَرَ بْنَ الْخَطَّابِ، قَالَ وَهُوَ مُسْنِدٌ ظَهْرَهُ إِلَى الْكَعْبَةِ مَنْ أَخَذَ ضَالَّةً فَهُوَ ضَالٌّ .
also referenced under: Book 36, Hadith 1453 (In-Book)