Musannaf Ibn Abi Shayba 3:5978

Waki’ narrated to us, from al-A’mash and Sufyan, from Zubayd, from Murrah, who said: Abdullah said: "Indeed, as long as you are in prayer, you are knocking at the door of the King. And whoever knocks frequently at the door of the King, it is likely that it will be opened for him."