Benefits of Duaa

Abu Hurairah  narrates from Rasool-Allah , "When anyone of you does duaa then don't say "Oh Allah! Forgive me if you want", "Have mercy on me if you want", "Give me subsistence if you wish" rather believe completely that HE will do whatever HE wishes. Nobody can force HIM." (Bukhari)
Abu Hurairah  narrates from Rasool-Allah , "Allah excepts the duaa of a person till the duaa is not related to sin or breaking of some relation and until he does not haste in that duaa. The people (May Allah be happy with them) asked, "Oh Rasool-Allah! What is meant by haste?" He  replied, "That the person say "I did duaa again and again but my duaa wasn't accepted and afterwards he becomes hopeless and leaves the duaa. This is called haste." (Muslim)

Jabir  narrates from Rasool-Allah , "Don't do Bad-duaa [against] yourself, your wealth, or for your children. May it be the moment of acceptance in the court of Allah and your Bad-duaa be accepted. (Muslim)
Boraidah  narrates that Rasool-Allah  heard a man performing duaa like so *1, so He  said, "He [the man] has performed duaa with the Ism-e-Azam and when Allah is asked through the Ism-e-Azam, Allah grants and when duaa is performed with it HE accepts it." (Tirmizi and Abu Dawood)
Asma binte Yazeed (Radi Allahu Anha) narrates from Rasool-Allah , "Ism-e-Azam is in these two ayahs  and in the beginning of Ale-Imran . " (Tirmizi and Abu Dawood)
Abu Hurairah  narrates from Rasool-Allah , "Saying  is dearer to me than this world and all that is in it." (Mislim)
Abu Hurairah  narrates from Rasool-Allah , "Whoever said  hundred times in one day his sins are erased even if they are equal to the foam of the sea." (Muslim and Bukhari)
Abu Hurairah  narrates from Rasool-Allah , "Every Prophet had a special duaa which is accepted in the court of Allah. All Prophets were in quick in their duaas. On the day of Judgment, for the intercession of my Ummah, I have saved my duaa. My duaa will reach to every person in my ummah, who did not make any partners with Allah. (Muslim)
*1