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 Messenger of Allah  (صلى الله عليه وآله وسلم) said:‘Verily Allah the Almighty has prescribed the obligatory deeds, so do not neglect them; He has set certain limits, so do not go beyond them; He has forbidden certain things, so do not indulge in them; and He has said nothing about certain things, as an act of mercy to you, not out of forgetfulness, so do not go enquiring into these.‘"

[It was narrated on the authority of Abi Tha'labah Al-Khushani Jurthum bin Nashir, radiyallahu 'anhu - Related by Al-Daraqutni and others - Al-Nawawi said this is a hasan (fine) hadith]

" إِنَّ اللهَ تَعَالَى فَرَضَ فَرَائِضَ فَلاَ تُضَيِّعُوهَا، وَحَدَّ حُدُوداً فَلاَ تَعْتَدُوهَا، وَحَرَّمَ أَشْيَاءَ فَلاَ تَنْتَهِكُوهَا، وَسَكَتَ عَنْ أَشْيَاءَ رَحْمَةً لَكُمْ غَيْرَ نِسْيَانٍ فَلاَ تَبْحَثُوا عَنْهَا"
[حَدِيثٌ حَسَنٌ رَوَاهُ الدَّارَقُطْنِي وَغَيْرُهُ]