Musnad Ahmad 30:1555

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah and Ishaq ibn 'Isa narrated to us, they said: Ibn Lahi'ah narrated to us, Yazid ibn Abi Habib narrated to us, from Lahi'ah ibn 'Uqbah, from Abu al-Ward—Ishaq al-Madini said: From Abu Hurayrah, he said: I heard the Prophet, peace and blessings of Allah be upon him, saying: "Beware of the *munaffalah* (stray/unattended) horses, for if they encounter [the enemy], they flee, and if they obtain spoils, they embezzle [them]."