Shama-il Tirmidhi Chapter 42, Hadith Number 15 (295).

Abu Salih R.A. reports “I enquired from Aayeshah and Ummi Salamah Radhiyallahu anhuma that which act was the most beloved by Rasulullah Sallallahu Alayhi Wasallam?” Both gave the reply “That deed which was practised continuously, even if it was a little”.

The object of all these ahadeeth are that saum, and likewise all other nafl deeds, even if it be a little, or whatever could be done should be practised continuously and with care. One should not forsake these with the thought that it cannot be practised constantly because the nawafil are the only acts that compensate the short comings of the faraa’id (compulsory act). Therefore one should endeavour to observe and practise as much as one possible can.