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".
English reference: Book 42, Hadith 295
Arabic reference: Book 43, Hadith 312