Surah Al Hajj Tafseer

And if they argue with you, say:"Allah knows best what you do."

This is like the Ayah:

وَإِن كَذَّبُوكَ فَقُل لِّى عَمَلِى وَلَكُمْ عَمَلُكُمْ أَنتُمْ بَرِييُونَ مِمَّأ أَعْمَلُ وَأَنَاْ بَرِىءٌ مِّمَّا تَعْمَلُونَ

And if they belie you, say:"For me are my deeds and for you are your deeds! You are innocent of what I do, and I am innocent of what you do!" (10:41)

اللَّهُ أَعْلَمُ بِمَا تَعْمَلُونَ

Allah knows best of what you do.

This is a stern warning and definite threat, as in the Ayah:

هُوَ أَعْلَمُ بِمَا تُفِيضُونَ فِيهِ كَفَى بِهِ شَهِيداً بَيْنِى وَبَيْنَكُمْ

He knows best of what you say among yourselves concerning it! Sufficient is He as a witness between me and you! (46:8)

Allah says here:

اللَّهُ يَحْكُمُ بَيْنَكُمْ يَوْمَ الْقِيَامَةِ فِيمَا كُنتُمْ فِيهِ تَخْتَلِفُونَ