How to swear?

Narrated Talha bin 'Ubaidullah:

A man came to Allah's Apostle asking him about Islam, Allah's Apostle ﷺ said, "You have to offer five compulsory prayers in a day and a night (24 hours)." The man asked, "Is there any more compulsory prayers for me?" Allah's Apostle ﷺ said, "No, unless you like to offer Nawafil (i.e. optional prayers)." Allah's Apostle then added, "You have to observe fasts during the month of Ramadan." The man said, "Am I to fast any other days?' Allah's Apostle ﷺ said, "No, unless you wish to observe the optional fast voluntarily." Then Allah's Apostle told him about the compulsory Zakat. The man asked, "Do I have to give anything besides?" Allah's Apostle ﷺ said, "No, unless you wish to give in charity voluntarily." So, the man departed saying, "By Allah I will neither do more nor less than that." Allah's Apostle ﷺ said, "If he has said the truth he will be successful."

حَدَّثَنَا إِسْمَاعِيلُ بْنُ عَبْدِ اللَّهِ، قَالَ حَدَّثَنِي مَالِكٌ، عَنْ عَمِّهِ أَبِي سُهَيْلٍ، عَنْ أَبِيهِ، أَنَّهُ سَمِعَ طَلْحَةَ بْنَ عُبَيْدِ اللَّهِ، يَقُولُ جَاءَ رَجُلٌ إِلَى رَسُولِ اللَّهِ ﷺ فَإِذَا هُوَ يَسْأَلُهُ عَنِ الإِسْلاَمِ، فَقَالَ رَسُولُ اللَّهِ ﷺ " خَمْسُ صَلَوَاتٍ فِي الْيَوْمِ وَاللَّيْلَةِ ". فَقَالَ هَلْ عَلَىَّ غَيْرُهَا قَالَ " لاَ، إِلاَّ أَنْ تَطَّوَّعَ ". فَقَالَ رَسُولُ اللَّهِ ﷺ " وَصِيَامُ رَمَضَانَ ". قَالَ هَلْ عَلَىَّ غَيْرُهُ قَالَ " لاَ، إِلاَّ أَنْ تَطَّوَّعَ ". قَالَ وَذَكَرَ لَهُ رَسُولُ اللَّهِ ﷺ الزَّكَاةَ. قَالَ هَلْ عَلَىَّ غَيْرُهَا قَالَ " لاَ، إِلاَّ أَنْ تَطَّوَّعَ ". فَأَدْبَرَ الرَّجُلُ وَهْوَ يَقُولُ وَاللَّهِ لاَ أَزِيدُ عَلَى هَذَا وَلاَ أَنْقُصُ. قَالَ رَسُولُ اللَّهِ ﷺ " أَفْلَحَ إِنْ صَدَقَ ".