Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) said to Abbas: 'O uncle, shall I not give you a gift, shall I not benefit you, shall I not uphold my ties of kinship with you?' He said: 'Of course, O Messenger of Allah.' He said: 'Pray four Rakah, and recite in each Rakah the Opening of the Book (Al-Fatihah) and a Surah. When you have finished reciting, say: Subhan-Allah wal-hamdu Lillah wa la ilaha illallah wa Allahu Akbar (Glory is to Allah, praise is to Allah, none has the right to be worshipped but Allah and Allah is the Most Great) fifteen times before you bow in Ruku. Then bow and say it ten times; then raise your head and say it ten times; then prostrate and say it ten times; then raise your head and say it ten times; then prostrate and say it ten times; then raise your head and say it ten times before you stand up. That wil be seventy-five times in each Rakah and three hundred times in the four Rakah, and even if your sins are like the grains of sand, Allah will forgive you for them.' He said: 'O Messenger of Allah, what if someone cannot say it in one day?' He said: 'Then say it once in a week; if you cannot, then say it once in a month' until he said: 'Once in a year.'" Daif