Sayyiduna Uqbah bin Amir (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "The guarantee against a slave's defects lasts three nights."
Sa'id (may Allah have mercy on him) said: I asked Qatadah (may Allah have mercy on him): How is that? He said: If the buyer finds a defect in the goods and returns them within those three days, he will not even be asked for proof. But after the three days have passed, proof will be required that he bought it from him and the defect was present at that time; otherwise the seller will swear an oath that this disease was not in it at the time of sale.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10752