Abu Dawud Book 2, Hadith Number 1480.

Chapter : Supplications.

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.