ذَٰلِكَ لِيَعْلَمَ أَنِّى لَمْ أَخُنْهُ بِٱلْغَيْبِ وَأَنَّ ٱللَّهَ لَا يَهْدِى كَيْدَ ٱلْخَآئِنِينَ
Zaalika liya'lama annee lam akhunhu bilghaibi wa annal laaha laa yahdee kaidal khaaa'ineen
[Joseph said, ‘This was] for my master to know that I did not betray him behind his back: God does not guide the mischief of the treacherous.
That was so he would know I did not betray him in his absence, and that God does not guide the schemes of the treacherous.
Yusuf had refused to leave the prison until the women were questioned, so his name is cleared in the open, on the testimony of others, before he ever walks out of the cell. The word for scheming here, kayd, runs through the whole surah: the brothers plot, the women plot, and later God is said to have devised the plan that keeps Binyamin with Yusuf.