Malik related to me from Yahya ibn Said from Said ibn al-Musayyabthat Umar ibn al-Khattab said while he was leaning his back againstthe Kaba, "Whoever takes a stray is astray." (ID)
USC-MSA web (English) reference: Book 36, Hadith 50 (ID)
Arabic reference (ID): Book 36, Hadith 1453 (ID)