Narrated 'Aisha (Radhiallaho anha):
Once the Prophet (sallallaahu 'alaihi wasallam) 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."

Bukhari Vol. 1 : No. 41