Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: As long as you are in the state of prayer, you are knocking at the door of the King, and whoever keeps knocking at the door of the King abundantly, that door is eventually opened for him.
This narration was recorded by Imam al-Tabarani in al-Mu'jam al-Kabir; its narrators are the narrators of the "Sahih".