Abu Dawud Book 8, Hadith Number 2779.
Narated By A man from the Companions of the Prophet : Ubaydullah ibn Salman reported on the authority of a man from the Companions of the Prophet (PBUH): When we conquered Khaybar, they (the people) took out their spoils which contained equipment and captives. The people began to buy and sell their spoils. When the Apostle of Allah (PBUH) prayed, a man came to him and said: Apostle of Allah, I have gained today so much so that no one gained from this valley. He asked: Woe unto you, how much did you gain? He replied: I kept on selling and buying until I gained three hundred uqiyahs. The Apostle of Allah (PBUH) said: I tell you a man who gained better than you. He asked: What is that, Apostle of Allah? He replied! Two rak’ahs (of supererogatory prayer) after the (obligatory) prayer.