Sahih Adab al-Mufrad

It is narrated from Sayyiduna Abu Darda’ (may Allah be pleased with him) that he used to say to the people: “We recognize you better than animal doctors recognize animals. We have recognized the good and the bad among you. Your good ones are those from whom good is expected and from whose evil people are safe. And your bad ones are those from whom no good is expected and from whose evil people are not safe, and whose freed slave is not made a slave again.” (That is, after freeing a person, one should not keep making him work by reminding him of the favor.)