Sayyiduna Abu Mas'ud (may Allah be pleased with him) said: There was a man from the Ansar called Abu Shu'ayb, and he had a slave who was a butcher. He saw the Messenger of Allah (peace and blessings be upon him) and recognised (the signs of) hunger on his (peace and blessings be upon him) face, so he said to his slave: "Woe to you! Prepare food for me, for I want to invite the Prophet (peace and blessings be upon him) as the fifth of five men." He (the slave) prepared the food, then he came to the Prophet (peace and blessings be upon him) and invited him (peace and blessings be upon him) as the fifth, and a man followed them. When they reached the door, the Prophet (peace and blessings be upon him) said: "This man has followed us. If you wish, permit him, and if you wish, he will go back." He said: "No, O Messenger of Allah, rather I permit him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14602