This dua is very special to me. You can say that this is my favorite dua. I try to make it every time in prayer before the tasleem .
In the hadith of Ali [RA] on the description of how Allah’s Messenger [SAWS] used to pray, he reported that at the end of what he said between the tashahhud and the tasleem was:
Transliteration (hover your mouse for translation)
Allahumm-maghfirlee ma qaddamtu
wa ma akh-khartu
wa ma asrartu
wa ma aa’lantu
wa ma asraftu
wa ma anta aa’lamu bihi min-nee
wa antal mu-akh-khiru
la ilaha illa anta
Al Muqaddim – The One who hastens or speeds whatever He wills/The First
Al Muakh-khir – The One who delays or slows whatever He wills/The Last
We’re asking for forgiveness for everything: for things we did, things we’ll do, things we conceal, things we reveal, things we do over-excessively, and for things we don’t even know exist within us (e.g. the disease of arrogance that lurks amongst most of us, but most of us are sadly unaware of)…Ya Salaam! What an amazing dua!
Also, keep in mind that the Prophet [SAWS] made a similar dua for his ummah in EVERY prayer.
Memorize. This. Dua.