Mishkat al-Masabih 16
Talha b. ‘Ubaidallah said:
A man of the people of Najd with dishevelled hair came to God’s messenger. We could hear the sound of his voice, but could not understand what he was saying till he came near God’s messenger and we realised that he was asking about Islam. God’s messenger said, “Five times of prayer each day and night.” He asked, “Must I observe any more than them?” He replied, “No, unless you do it voluntarily.” God's messenger said, “And fasting during the month of Ramadan.” He asked, “Must I observe anything else?” He replied, “No, unless you do it voluntarily.” Talha said that God's messenger mentioned the zakat to him, and he asked, “Must I pay anything else?” He replied, “No, unless you do it voluntarily.” He said that the man turned away saying, “I swear by God that I shall not add anything to this or fall short of it.” So God’s messenger said, “The man will prosper if he is speaking the truth.” 

(Bukhari and Muslim.)
وَعَنْ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ قَالَ: جَاءَ رَجُلٌ إِلَى رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ مِنْ أَهْلِ نَجْدٍ ثَائِرُ الرَّأْسِ نَسْمَعُ دَوِيَّ صَوْتِهِ وَلَا نَفَقَهُ مَا يَقُولُ حَتَّى دَنَا مِنْ رَسُولِ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ فَإِذَا هُوَ يَسْأَلُ عَنِ الْإِسْلَامِ فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «خَمْسُ صَلَوَاتٍ فِي الْيَوْمِ وَاللَّيْلَةِ» . فَقَالَ: هَلْ عَلَيَّ غَيْرُهُنَّ؟ فَقَالَ: " لَا إِلَّا أَنْ تَطَّوَّعَ. قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: وَصِيَامُ شَهْرِ رَمَضَانَ ". قَالَ: هَلْ عَلَيَّ غَيْرُهُ؟ قَالَ: «لَا إِلَّا أَنْ تَطَّوَّعَ» . قَالَ: وَذَكَرَ لَهُ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ الزَّكَاةَ فَقَالَ: هَلْ عَلَيَّ غَيْرُهَا؟ فَقَالَ: " لَا إِلَّا أَنْ تَطَّوَّعَ. قَالَ: فَأَدْبَرَ الرَّجُلُ وَهُوَ يَقُولُ: وَاللَّهِ لَا أَزِيدُ عَلَى هَذَا وَلَا أَنْقُصُ مِنْهُ. فَقَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَفْلح الرجل إِن صدق»