Musnad Ahmad 482:67

'Abd al-Samad narrated to us, Hisham narrated to us, from Qatadah, from al-Hasan, from 'Uqbah ibn 'Amir, that the Messenger of Allah (ﷺ) said: "The warranty period for a slave is four nights." Qatadah said: "And the people of Medina say: three nights."