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." (IT)
USC-MSA web (English) reference: Book 36, Hadith 50 (IT)
Arabic reference (IT): Book 36, Hadith 1453 (IT)