Abu Dawud Book 16, Hadith Number 3380.

Narated By Hakim ibn Hizam : The Apostle of Allah (PBUH) 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 (PBUH). The Prophet (PBUH) gave it as alms (sadaqah) and invoked blessing on him in his trading.