From Ibn 'Abbas (RA), that the Prophet (SAW) said:
"O 'Abbas! O my uncle! Shall i not give you, shall i not bestow upon you, shall i not donate to you? Shall i not produce in you ten things, if you do that, Allah(SWT) will forgive your sins, first and last, old & new mistakes and deliberate errors, small & great, hidden & open? Ten things. You should pray four rakahs, reciting in each rak'ah the Opening of the Book (Al-Fatihah) and a surah (of your choice). When you have finished reciting this in the first rak'ah, say this supplication while standing 15 times: 'SUBHANALLAH, AL-HAMDULILLAH, LA ILAHA ILLALLAH, ALLAHU AKBAR'. When you go on rukoo', say it 10 times, raise from rukoo' and say it 10 times. Prostrate and say it while in prostration (10 times), raise from prostration and say it 10 times. (Remember sujood is done twice). These add up to 75 times in every rak'ah, which you do in four rak'ahs (300 times).
So if your sins were like the foam of the sea or like heaped sand, Almighty Allah would forgive you. If you are able to pray it once everyday, do so. If not, once in every year. If not, once in one's lifetime."
May Almighty Allah assist us. Ameen...

Please the OoP,is what you wrote above is a hadith? If so,which hadith is it?