Hazrat Abdullah bin Abbas (Radhiallaahu Anhu) reports that the Messenger of Allah (Sallallaahu Alayhi Wasallam) said to his uncle Abbas ibn Abdul Muttalib (Radhiallaahu Anhu), "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? What you have to do is offer four rak'ah of optional prayers in this way: in every 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 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."