Hadith 514520
Chapter 36: Judgements - كتاب الأقضية
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 .
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."
Reference Muwatta Malik 514520
USC-MSA web (English) reference : Book 36, Hadith 50