Musnad Abu Ya'la al-Mawsili 209:91

Abu Khaythama narrated to us, Waki' narrated to us, Sufyan narrated to us, from Abu Ishaq, from Abu Salamah bin 'Abd al-Rahman, from Umm Salamah, who said: "And the most beloved of deeds to the Messenger of Allah, may Allah bless him and grant him peace, was that which was done consistently, even if it were little."