The Prophet ﷺ sent some horsemen to Najd and they brought a man called Thumama bin Uthal from Bani Hanifa. They fastened him to one of the pillars of the mosque. The Prophet ﷺ came and ordered them to release him. He went to a (garden of) date-palms near the mosque, took a bath and entered the mosque again and said, "None has the right to be worshipped but Allah and Muhammad is His Apostle" (i.e. he embraced Islam).  

بَعَثَ النَّبِيُّ ﷺ خَيْلاً قِبَلَ نَجْدٍ فَجَاءَتْ بِرَجُلٍ مِنْ بَنِي حَنِيفَةَ يُقَالُ لَهُ ثُمَامَةُ بْنُ أُثَالٍ فَرَبَطُوهُ بِسَارِيَةٍ مِنْ سَوَارِي الْمَسْجِدِ فَخَرَجَ إِلَيْهِ النَّبِيُّ ﷺ فَقَالَ أَطْلِقُوا ثُمَامَةَ فَانْطَلَقَ إِلَى نَخْلٍ قَرِيبٍ مِنَ الْمَسْجِدِ فَاغْتَسَلَ ثُمَّ دَخَلَ الْمَسْجِدَ فَقَالَ أَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ