Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4617
'Urwat al-Bariqi reported Allah's Messenger (may peace be upon him) having said this: Good is tied to the forelock of the horses. It Was said to him: Messenger of Allah, why is it so? He (the Holy Prophet said): For reward and booty until the Day of Judgment.