It is narrated from Sayyiduna Abdullah bin Amr (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "There are three people whose prayer is not accepted: one who leads a people in prayer while they dislike him; a man who comes to the prayer 'dibaran' (and dibar is that he comes after its time); and a man who enslaves again the slave he has freed."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13773