Sahih Bukhari Volume 007, Book 065, Hadith Number 371.
Narrated By Abu Mas'ud Al-Ansari : There was an Ansari man nicknamed, Abu Shu'aib, who had a slave who was a butcher. He came to the Prophet while he was sitting with his companions and noticed the signs of hunger on the face of the Prophet. So he went to his butcher slave and said, "Prepare for me a meal sufficient for five persons so that I may invite the Prophet along with four other men." He had the meal prepared for him and invited him. A (sixth) man followed them. The Prophet said, "O Abu Shu'aib! Another man has followed us. If you wish, you may invite him; and if you wish, you may refuse him." Abu Shu'aib said, "No, I will admit him."