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"Verily, your Lord knows that you do stand (to pray at night) a little less than two-thirds of the night, or half the night, or a third of the night, and so do a party of those with you, And Allaah measures the night and the day. He knows that you are unable to pray the whole night, so He has turned to you (in mercy). So, recite you of the Qur'aan as much as may be easy for you. He knows that there will be some among you sick, others travelling through the land, seeking of Allaah's Bounty; yet others fighting in Allaah's Cause. So recite as much of the Qur'aan as may be easy (for you), and perform As-Salaat (Iqaamat-as-Salaat) and give Zakaat, and lend to Allaah a goodly loan, and whatever good you send before you for yourselves, (i.e. Nawaafil non-obligatory acts of worship: prayers, charity, fasting, Hajj and 'Umrah, etc.), you will certainly find it with Allaah, better and greater in reward. And seek Forgiveness of Allaah. Verily, Allaah is Oft-Forgiving, Most-Merciful." [Al-Muzzammil (73):20]

Narrated 'Aa'ishah:

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." [The Book of Faith Volume 1, Book 2, Hadeeth 41]