Mishkat al-Masabih 13 Marriage كتاب النكاح 236

وَعَنِ ابْنِ عَبَّاسٍ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَمَرَ رَجُلًا حِينَ أَمَرَ الْمُتَلَاعِنَيْنِ أَنْ يَتَلَاعَنَا أَنْ يَضَعَ يَدَهُ عِنْدَ الْخَامِسَةِ عَلَى فِيهِ وَقَالَ: «إِنَّهَا مُوجِبَةٌ» . رَوَاهُ النَّسَائِيُّ

Ibn ‘Abbas said that when the Prophet ordered a man and wife to invoke curses regarding one another, he ordered a man to put his hand on his mouth when he came to the fifth utterance, saying that it would be the deciding one. Nasa’i transmitted it.