Thawban narrated that the Prophet ﷺ said:

"When one of you suffers from fever - and indeed fever is a piece of the Fire - let him extinguish it with water. Let him stand in a flowing river facing the direction of it and say: Allahummasahfi 'abdaka wa saddik Rasulak ('In the name off Allah. O Allah! Cure your slave and testify to Your Messenger.)' Doing so after Salat As-Subh(Fajr) and before the rising of the sun. Let him submerse himself in it three times, for three days. If he is not cured in three, then five. If he is not cured in five, then seven. If he is not cured in seven, then nine. For indeed it will not remain after nine, with the permission of Allah."

حَدَّثَنَا أَحْمَدُ بْنُ سَعِيدٍ الأَشْقَرُ الرِّبَاطِيُّ، حَدَّثَنَا رَوْحُ بْنُ عُبَادَةَ، حَدَّثَنَا مَرْزُوقٌ أَبُو عَبْدِ اللَّهِ الشَّامِيُّ، حَدَّثَنَا سَعِيدٌ، رَجُلٌ مِنْ أَهْلِ الشَّامِ أَخْبَرَنَا ثَوْبَانُ، عَنِ النَّبِيِّ ﷺ قَالَ " إِذَا أَصَابَ أَحَدَكُمُ الْحُمَّى فَإِنَّ الْحُمَّى قِطْعَةٌ مِنَ النَّارِ فَلْيُطْفِئْهَا عَنْهُ بِالْمَاءِ فَلْيَسْتَنْقِعْ نَهْرًا جَارِيًا لِيَسْتَقْبِلَ جَرْيَةَ الْمَاءِ فَيَقُولُ بِسْمِ اللَّهِ اللَّهُمَّ اشْفِ عَبْدَكَ وَصَدِّقْ رَسُولَكَ بَعْدَ صَلاَةِ الصُّبْحِ قَبْلَ طُلُوعِ الشَّمْسِ فَلْيَغْتَمِسْ فِيهِ ثَلاَثَ غَمَسَاتٍ ثَلاَثَةَ أَيَّامٍ فَإِنْ لَمْ يَبْرَأْ فِي ثَلاَثٍ فَخَمْسٌ وَإِنْ لَمْ يَبْرَأْ فِي خَمْسٍ فَسَبْعٌ فَإِنْ لَمْ يَبْرَأْ فِي سَبْعٍ فَتِسْعٌ فَإِنَّهَا لاَ تَكَادُ تُجَاوِزُ تِسْعًا بِإِذْنِ اللَّهِ " .

قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ غَرِيبٌ .