Salat must be dedicated to God alone

Every facet of our religious practices must be devoted to God alone.

[6:162] Say, "My Contact Prayers (Salat), my worship practices, my life and my death, are all devoted absolutely to GOD alone, the Lord of the universe.

قُلْ إِنَّ صَلَاتِى وَنُسُكِى وَمَحْيَاىَ وَمَمَاتِى لِلَّهِ رَبِّ ٱلْعَـٰلَمِينَ

In theory, any place where we pray can be a place of worship, and God commands that we not call upon any other entity at any place of worship. Therefore, we should never call upon any other entity in any aspect of our worship.

[72:18] The places of worship belong to GOD; do not call on anyone else beside GOD.

وَأَنَّ ٱلْمَسَـٰجِدَ لِلَّهِ فَلَا تَدْعُوا۟ مَعَ ٱللَّهِ أَحَدًا

Yet, Sunnis do just that in their tashahud when they call directly upon the prophet as if he is able to hear their prayers.

“Salutations to God and prayers and good deeds. Peace be upon you, O Prophet, as well as God’s mercy and His blessings. Peace be upon us and upon the righteous servants of God. I bear witness that there is no deity but God, and I bear witness that Muhammad is His servant and His messenger.”

ٱلتَّحِيَّاتُ لِلَّٰهِ وَٱلصَّلَوَاتُ وَٱلطَّيِّبَاتُ، ٱلسَّلَامُ عَلَيْكَ أَيُّهَا ٱلنَّبِيُّ وَرَحْمَةُ ٱللَّٰهِ وَبَرَكَاتُهُ، ٱلسَّلَامُ عَلَيْنَا وَعَلَىٰ عِبَادِ ٱللَّٰهِ ٱلصَّالِحِينَ، أَشْهَدُ أَنْ لَا إِلَٰهَ إِلَّا ٱللَّٰهُ، وَأَشْهَدُ أَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ.

at-taḥiyyātu li-llāhi, wa-ṣ-ṣalawātu wa-ṭ-ṭayyibātu. as-salāmu ʿalayka ʾayyuhā n-nabiyyu wa raḥmatu -llāhi wa barakātuhu. as-salāmu ʿalaynā wa ʿalā ʿibādi -llāhi ṣ-ṣāliḥīna. ʾashhadu ʾan lā ʾilāha ʾillā -llāhu wa ʾashhadu ʾanna muḥammadan ʿabduhu wa rasūluhū.

The use of “Peace be upon you, O Prophet” ( ٱلسَّلَامُ عَلَيْكَ أَيُّهَا ٱلنَّبِيُّ ) is a direct call upon the prophet of God, who is not only another entity beside God but also dead and incapable of hearing their supplications.