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]

The Holy Messenger of Allah the Almighty صلي الله عليه و سلم is quoted on the authority of Abu Huraira رضی الله تعلٰی عنه to have said:
“Allah the Almighty says, ‘I am towards My servant as he is towards Me.  I am with him when he remembers Me (dhikr).  If he remembers Me within himself, I remember him within Myself.  If he remembers Me in a gathering, I remember him in a gathering better than it.’”

(Sahih Al-Bhukhari V2, P1101) [src] p5, 4 Natijatu Al-Fikr Fi Al-Jahri Fi Al-Dhikr (Outcome of Contemplation over Vociferous Dhikr) an extract from Al-Hawi Lil Fatawa  by Jalal Al-Din Al-Suyuti رحمتاالله علیه, Dar-ul-Ehsan Publications