AhsanAllahu ilaykum A brother at my Masjid always says رَبَّنَا وَلَكَ الْحَمْدُ وَشكر after the imam reciting سَمِعَ اللَّهُ لِمَنْ حَمِدَهُ. Is this authentic?

Ustadh AbdulAziz al-Haqqan

The addition ashukr is not authentic, so it is always better as imam ibn baz said not to say it.