Sahih Bukhari Volume 001, Book 012, Hadith Number 819.
Narated By Anas bin Malik : My grandmother Mulaika invited Allah's Apostle for a meal which she had prepared specially for him. He ate some of it and said, "Get up. I shall lead you in the prayer." I brought a mat that had become black owing to excessive use and I sprinkled water on it. Allah's Apostle stood on it and prayed two Rakat; and the orphan was with me (in the first row), and the old lady stood behind us.