كتاب صلاة المسافرين وقصرها
6  
The Book of Prayer - Travellers
(30)
Chapter: The virtue of a deed that is done persistently, whether it be Qiyam al-Lail or anything else. The command to be moderate in worship, which means adopting what one can persist in. The command to the one who gets tired or weary when praying to stop until that feeling passes
(30)
باب فَضِيلَةِ الْعَمَلِ الدَّائِمِ مِنْ قِيَامِ اللَّيْلِ وَغَيْرِهِ ‏
Sahih Muslim 783 b

'A'isha reported Allah's Messenger (ﷺ) as saying:

The acts most pleasing to Allah are those which are done continuously, even if they are small. and when 'A'isha did any act she did it continuously.
وَحَدَّثَنَا ابْنُ نُمَيْرٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا سَعْدُ بْنُ سَعِيدٍ، أَخْبَرَنِي الْقَاسِمُ بْنُ مُحَمَّدٍ، عَنْ عَائِشَةَ، قَالَتْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ أَحَبُّ الأَعْمَالِ إِلَى اللَّهِ تَعَالَى أَدْوَمُهَا وَإِنْ قَلَّ ‏"‏ ‏.‏ قَالَ وَكَانَتْ عَائِشَةُ إِذَا عَمِلَتِ الْعَمَلَ لَزِمَتْهُ ‏.‏
Reference : Sahih Muslim 783b
In-book reference : Book 6, Hadith 258
USC-MSA web (English) reference : Book 4, Hadith 1713
  (deprecated numbering scheme)