The defendant has to take an oath wherever it becomes legally compulsory

قَضَى مَرْوَانُ بِالْيَمِينِ عَلَى زَيْدِ بْنِ ثَابِتٍ عَلَى الْمِنْبَرِ فَقَالَ أَحْلِفُ لَهُ مَكَانِي. فَجَعَلَ زَيْدٌ يَحْلِفُ، وَأَبَى أَنْ يَحْلِفَ عَلَى الْمِنْبَرِ، فَجَعَلَ مَرْوَانُ يَعْجَبُ مِنْهُ. وَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «شَاهِدَاكَ أَوْ يَمِينُهُ». فَلَمْ يَخُصَّ مَكَانًا دُونَ مَكَانٍ.Marwan ordered Zaid bin Thabit to take an oath on the pulpit, but the latter said, “I will take an oath at my place,” and started taking the oath and refused to take it on the pulpit. Marwan was surprised at his refusal. The Prophet ﷺ said to the plaintiff, “Produce your two witnesses, or else the defendant has to take an oath.” But he did not specify (where the oath was to be taken).