It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Indeed your Lord is Modest and Generous; He is too modest, when a servant raises his two hands, to return them empty, with no good in them. So when one of you raises his hands, let him say: 'O Ever-Living One, there is no god worthy of worship but You, O Most Merciful of those who show mercy,' three times. Then when he lowers his hands, let him wipe that good over his face."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13500