Ash-Shama'il Al-Muhammadiyah, Book 42, Hadith 295 (English) ⓘ

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".

حَدَّثَنَا أَبُو هِشَامٍ مُحَمَّدُ بْنُ يَزِيدَ الرِّفَاعِيُّ، قَالَ‏:‏ حَدَّثَنَا ابْنُ فُضَيْلٍ، عَنِ الأَعْمَشِ، عَنْ أَبِي صَالِحٍ، قَالَ‏:‏ سَأَلْتُ عَائِشَةَ، وَأُمَّ سَلَمَةَ، أَيُّ الْعَمَلِ كَانَ أَحَبَّ إِلَى رَسُولِ اللهِ صلى الله عليه وسلم‏؟‏ قَالَتَا‏:‏ مَا دِيمَ عَلَيْهِ، وَإِنْ قَلَّ‏.‏