Among the optional (nafl) prayers, this prayer is of great magnitude.
Hazrat Abdullah bin Abbas (may Allah be pleased with him) reports that the Messenger of Allah (Allah bless him and give him peace) said to his uncle Abbas ibn Abdul Muttalib (may Allah be pleased with him),
“Shall I not give you a gift?
Shall I grant you a blessing?
Shall I inform you of a great benefit?
Shall I give you something, which if you do, Allah will forgive you all your sins, old and past and new, committed by mistake or knowingly, small and big, committed in secret or openly?
It is that you have to offer four unit (rak’ah) of optional prayers in this way: in every unit (rak’ah) after the Al-Hamd and Surah and while yet standing, recite the third kalimah
fifteen times, then in the bowing (ruku), ten times, then standing up after the bow (qoma), ten times, then in each prostration ten times and in between the two prostrations ten times, and sitting up after the second prostration, ten times; seventy-five times per unit (rak’ah), three-hundred times in all.
Then, He said, “If possible, offer this prayer once every day; if you cannot, offer it once on Fridays, if this is not possible, offer it once a month, if this too is not possible, offer it once a year, and if even this is not possible, offer it once in your lifetime.”
(Ibn Maja, Tirmizi, Abu Daud)
Hazrat Abbas (Radhiallaahu Anhu) used to offer this prayer every Friday.
Another way of offering this prayers (may be easier for Hanafis) has been reported by Hadhrat Abdullah bin Mubarak (may Allah have mercy on him). It is to recite the kalima fifteen times after opening praise (thana) but before al-fathiha (and in all raka’h). The ten times after al-fathiha and a Surah. The rest as stated above, except that there will be no need to recite these kalimat after the second prostration (sujud, in sitting position) in any rak’ah.