Sayyiduna Ammar bin Yasir (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Whoever beats his slave unjustly, retribution will be exacted from him for it on the Day of Resurrection."
Al-Tabarani recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العتق / 7232