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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 Ibn Mas'ud:

The Prophet used to take care of us in preaching by selecting a suitable time, so that we might not get bored. (He abstained from pestering us with sermons and knowledge all the time). [The Book of Knowledge Volume 1, Book 3, Hadeeth 68]