Abu Dawud
Prayer
Book Number: 002 / Hadith Number: 1480
Narated By Abdullah ibn Abbas : The Prophet (pbuh) said: Do not cover the walls. He who sees the letter of his brother without his permission, sees Hell-fire.
Supplicate Allah with the palms of your hands; do not supplicate Him with their backs upwards. When you finish supplication, wipe your faces with them.