Sahih Bukhari Volume 001, Book 002, Hadith Number 041.
Sahih Bukhari Book 02. Belief
Narated By 'Aisha : Once the Prophet came while a woman
was sitting with me. He said, "Who is she?" I replied, "She is so
and so," and told him about her (excessive) praying. He said
disapprovingly, "Do (good) deeds which is within your capacity
(without being overtaxed) as Allah does not get tired (of giving
rewards) but (surely) you will get tired and the best deed (act
of Worship) in the sight of Allah is that which is done
regularly."