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 ‏

'A'isha is reported to have said that the Messenger of Allah (ﷺ) was asked about the act most pleasing to Allah. He replied:

That which is done continuously, even if it is small.
حَدَّثَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، حَدَّثَنَا مُحَمَّدُ بْنُ جَعْفَرٍ، حَدَّثَنَا شُعْبَةُ، عَنْ سَعْدِ بْنِ إِبْرَاهِيمَ، أَنَّهُ سَمِعَ أَبَا سَلَمَةَ، يُحَدِّثُ عَنْ عَائِشَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم سُئِلَ أَىُّ الْعَمَلِ أَحَبُّ إِلَى اللَّهِ قَالَ ‏ "‏ أَدْوَمُهُ وَإِنْ قَلَّ ‏"‏ ‏.‏
Reference : Sahih Muslim 782 b
In-book reference : Book 6, Hadith 256
USC-MSA web (English) reference : Book 4, Hadith 1711
  (deprecated numbering scheme)