Sayyiduna Abu al-Darda (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) said: "Whoever gives a divorce, or frees a slave, by way of play (that is, in jest), it will be counted just as he said."
Imam al-Tabarani recorded this narration; its chain contains a narrator, Isma'il bin Muslim al-Makki, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العتق / 7274