Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates from the Prophet (peace and blessings be upon him): "Whoever frees his share in a slave and also has wealth, let him have him freed completely. If he has no wealth, the slave is to be made to work, but without being overburdened."
Al-Hajjaj bin al-Hajjaj, Aban bin Yazid al-Attar and Musa bin Khalaf al-Ammi likewise narrated it from Qatadah, mentioning the istis'a' (making the slave work) inserted (mudraj) into the hadith. As for al-Shafi'i (may Allah have mercy on him), he declared the matter of the slave's working in it weak for several reasons, among them that Shu'bah bin al-Hajjaj and Hisham al-Dastuwa'i narrated this hadith from Qatadah without the istis'a' in it, and they are the better memorisers.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21372