Majma al-Zawaid — Hadith 6858

Abu Amr al-Shaybani narrates: I brought runaway slaves from Yemen to Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), and he said: "There is a reward and also a gain." The narrator says: I said: "As for the reward, we know about it; what is meant by the gain?" He said: "Forty dirhams for each slave (as a bounty)."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Abu Riyah, whom I do not know, and the remaining narrators of this report are the narrators of the Sahih.