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Hisn al-Muslim (Fortress of the Muslim) 258
وَقَالَ صلى الله عليه وسلم: ((أَيَعْجِزُ أَحَدُكُم أَنْ يَكْسِبَ كُلَّ يَوْمٍ أَلْفَ حَسَنَةٍ)) فَسَأَلَهُ سَائِلٌ مِنْ جُلَسَائِهِ كَيْفَ يَكْسِبُ أَحَدُنَا أَلْفَ حَسَنَةٍ؟ قَالَ: ((يُسَبِّحُ مِائَةَ تَسْبِيحَةٍ، فَيُكتَبُ لَهُ أَلْفُ حَسَنَةٍ أَوْ يُحَطُّ عَنْهُ أَلْفُ خَطِيئَةٍ)).
(Sa’d said: ‘We were sitting with the Messenger of Allah , and he said: ‘Are any of you unable to gain a thousand good deeds each day?’ Somebody then asked him : How does one achieve a thousand good deeds? He replied: ‘He should say: (How perfect Allah is.) one hundred times, for a thousand good deeds are recorded for him or a thousand bad deeds are wiped away.)
Cited in Hisn al-Muslim (Fortress of the Muslim) (No. 258).