Commercial Transactions (Kitab Al-Buyu)
Dawud :: Book 22 : Hadith 3380
Narrated Hakim ibn Hizam:
The Apostle of Allah (peace_be_upon_him) sent with him a dinar to buy a sacrificial animal for him. He bought a sheep for a dinar, sold it for two and then returned and bought a sacrificial animal for a dinar for him and brought the (extra) dinar to the Prophet (peace_be_upon_him). The Prophet (peace_be_upon_him) gave it as alms (sadaqah) and invoked blessing on him in his trading.