A Supplication Whose Reward Is Kept Secret

Sayyiduna Abdullah bin Umar (Radi Allahu Anhuma) narrates that the beloved Prophet (SallAllahu Alayhi wa Sallam) told them:

"One of the slaves of Allah said: 

يا رب لك الحمد كما ينبغي لجلال وجهك ولعظيم سلطانك
Ya Rabb! Lakal-hamdu kama yambagi li-jalali Wajhika wa li 'azeema sultanika 

The angels were uncertain and did not know how to write (the reward of) this down, so they ascended to heaven and said: 'O our Lord, Your slave has said a word that we do not know how to write (its reward) down.' Allah said - and He knows best what His slave said - 'What did My slave say?' They said: 'O Lord, he said "Ya Rabb! Lakal-hamdu kama yanbagi li-jalali Wajhika wa li'azeema sultanika" Allah said to them: 'Write it down as My slave said it, until he meets Me and I shall reward him for it.'" 

[Sunan Ibn Majah, Vol. 2, Page 1249, Hadith 3801]