Allah Most High says: "He who is hostile to a friend of Mine I declare war against. My slave approaches Me with nothing more beloved to Me than what I have made obligatory upon him, and My slave keeps drawing nearer to Me with voluntary works until I love him. And when I love him, I am his hearing with which he hears, his sight with which he sees, his hand with which he seizes, and his foot with which he walks. If he asks me, I will surely give to him, and if he seeks refuge in Me, I will surely protect him"

(Sahih Bukhari - Fath al-Bari, 11.340 41, hadith 6502) [src: suhba.org]

Narrated by Ubayy ibn Ka'b (Radi Allah Ta'lah Anhu): "I said: Allah's Messenger (Sal-allahu-aleihi-wasallam), I invoke blessings upon you very frequently. How much of my supplication should I devote to you. He said: As much as you like. I said: May it be a quarter? He said: As much as you like, but if you increase it, it will be good for you. I said: May it be a half? He said: As much as you like, but if you increase it, it will be good for you. I said: May it be two thirds? He said: As much as you like but if you make an increase in it, it will be better. I said: May I devote the whole of my supplication to you (that I make in my prayer)? Thereupon he said: In that case it will take care of you and your sins would be expiated."

Tirmidhi transmitted it. Hadith No. 929 of Mishkat al-Masabih. [src: road-to-heaven.com]