Abdullah bin Mas'ud (may Allah be pleased with him) said: As long as you are in prayer, it is as though you are knocking at the door of a king; and the more one knocks at the door of a king, the door is opened for the one who knocks.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4569