-اَللّٰهُمَّ اغْفِرْلِيْ مَا قَدَّ مْتُ وَ مَا أَخَّرْ تُ، وَ مَا أَسْرَ رْ تُ وَ مَا أَعْلَنْتُ، وَ مَا أَنْتَ أَ عْلَمُ بِهٖ مِنِّيْ

Allahummagh-fir lii maa qaddamtu wa maa akh-khart(u), wa maa asrartu wa maa a’lant(u), wa maa Anta a’lamu bihii minnii. (‘Abdullah ibn ‘Umar. Mustadrak Hakim #134/1934)

O Allah, forgive what I did earlier and what I did later; what I did secretly and what I did publicly; and whatever you know more than I.

Ameen

According to Sayyidnaa ‘Ali (ra) Prophet Muhammad (sallallahu alayhi wa sallem) used to say this du’aa after finishing the salaat.

(Source: Khalid Baig’s commentary to du’aa #59 in the Accepted Whispers: Munajat-e-Maqbul)