Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "No charity is more virtuous than charity given to a slave who is with a bad master."
Al-Tabarani recorded it in al-Mu'jam al-Awsat.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العتق / 7230