Zayd bin Aslam narrated from his father that he said: Abu Bakr (may Allah be pleased with him) took charge in the first year, and he distributed among the people equally; each person received ten dirhams. Then he distributed in the second year, and they received twenty dirhams each, and a few dirhams were left over with him. He addressed the people and said: O people, a few dirhams of this wealth are left over. You have servants who toil for you and do your work; if you wish, we will give them a small gift. They said: Do so. So he gave each person five dirhams. If this report is sound, it shows the reason for which he distributed to slaves: it was a small gift given with their masters' permission, so it is as if he gave it to their masters.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12983