The meaning of the second part of the testimony of faith, or Shahadah is:

لَا مَتْبُوعَ بِحَقٍّ إِلَّا رَسُولَ اللهِ صلى الله عليه وسلم

“There is none worthy of being followed in truth except the Messenger of Allah”

If someone does follow other than the Messenger of Allah in something which has no evidence to support it; then he has followed falsehood.

Allah the Exalted said:

اتَّبِعوا ما أُنزِلَ إِلَيكُم مِن رَبِّكُم وَلا تَتَّبِعوا مِن دونِهِ أَولِياءَ ۗ قَليلًا ما تَذَكَّرونَ

{Follow what has been revealed to you from your Lord, and do not follow other than Him any allies. Little do you remember}

[al-A’raf (7): 3]

He, the Exalted said:

فَلا وَرَبِّكَ لا يُؤمِنونَ حَتّىٰ يُحَكِّموكَ فيما شَجَرَ بَينَهُم ثُمَّ لا يَجِدوا في أَنفُسِهِم حَرَجًا مِمّا قَضَيتَ وَيُسَلِّموا تَسليمًا

{But no by your Lord, they will not truly believe until they make you [O Muhammad], judge concerning that over which they dispute among themselves and then find within themselves no discomfort from what you have judged and they submit in [full] submission}

[al-Nisa’ (4): 65]

And He, the Exalted said:

وَما كانَ لِمُؤمِنٍ وَلا مُؤمِنَةٍ إِذا قَضَى اللَّهُ وَرَسولُهُ أَمرًا أَن يَكونَ لَهُمُ الخِيَرَةُ مِن أَمرِهِم ۗ وَمَن يَعصِ اللَّهَ وَرَسولَهُ فَقَد ضَلَّ ضَلالًا مُبينًا

{It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice over the matter. And whoever disobeys Allah and His Messenger has certainly strayed into clear error}

[al-Ahzab (33): 36]

Muhammad b. ‘Abd al-Wahhab al-Wasabi al-‘Abdali, al-Qawl al-Mufid fi Adillat al-Tawhid, (San’a: Maktabat al-Irshad, 1427/2006), 35.