Narrated Abu Huraira:

The Prophet 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 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 an Muhammad is His Apostle (i.e. he embraced Islam)."

حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يُوسُفَ، قَالَ حَدَّثَنَا اللَّيْثُ، قَالَ حَدَّثَنَا سَعِيدُ بْنُ أَبِي سَعِيدٍ، سَمِعَ أَبَا هُرَيْرَةَ، قَالَ 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.