Al-Kāfi - Volume 2 > Balanced and Continuous Good Deeds
Hadith #6

6ـ عِدَّةٌ مِنْ أَصْحَابِنَا عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ عَنْ مُحَمَّدِ بْنِ إِسْمَاعِيلَ عَنْ جَعْفَرِ بْنِ بَشِيرٍ عَنْ عَبْدِ الْكَرِيمِ بْنِ عَمْرٍو عَنْ سُلَيْمَانَ بْنِ خَالِدٍ قَالَ قَالَ أَبُو عَبْدِ الله (عَلَيهِ السَّلام) إِيَّاكَ أَنْ تَفْرِضَ عَلَى نَفْسِكَ فَرِيضَةً فَتُفَارِقَهَا اثْنَيْ عَشَرَ هِلالاً.

6. A number of our people have narrated from Ahmad ibn Muhammad from Muhammad ibn Isma’il from Ja’far ibn Bashir from ‘Abd al-Karim ibn ‘Amr from Sulayman ibn Khalid who has said the following: “Abu ‘Abd Allah (a.s.) has said, ‘Be careful and do not give up on a good deed that you may have decided to do, before twelve lunar months end.’” Note: The good deeds and acts of worship in the above Ahadith refer to the optional acts of worship; the obligatory ones do not decrease or increase.