سنن النسائي

It was narrated that Aishah said: "The Messenger of Allah (ﷺ) did not refrain from (kissing) my forehead when he was fasting, and he did not die until most of his prayers were offered sitting down." Then she said something to the effect that (referred to the prayers) other than the obligatory prayers. "And the dearest of actions to him was that in which a person persists, even if it is little."