NEVER shall a man enter the house of his neighbor without permission, and this conduct may be learned, from the Holy One, blessed be He, who stood at the gate of the garden, and called to Adam [Gen. iii. 9]: "And the Lord God called unto the man, and said unto him, Where art thou?" It happened with the four elders Rabban Gamaliel, R. Jehoshua, R. Elazar b. Azariah, and R. Aqiba, who went to the interior cities of Rome, in one of which there lived a friend of theirs, a philosopher--R. Jehoshua asked Rabban Gamaliel if he would like to go and see their friend the philosopher, and he answered he would not. On the next morning, however, he asked him again, and he said he would. They then went, and R. Jehoshua knocked at the door of the philosopher, and at once the philosopher concluded that this must be the manner of a wise man. When he knocked again, the philosopher arose and washed his face, hands, and feet. When he knocked the third time, the philosopher opened the door, and saw that the sages of Israel were coming from both sides of the street; namely, Rabban Gamaliel being in. the centre, R. Jehoshua and R. Elazar b. Azariah to his right, and R. Aqiba to his left, and the philosopher was somewhat puzzled as to the manner of saluting them. He said within himself: "Whom of the sages shall I greet first? If I should greet R. Gamaliel first, I might thereby offend the other sages; and if I should address my greeting to all of them (without naming the head of them), I will offend R. Gamaliel." He therefore concluded to address them thus: "Peace to you, sages of Israel, and to R. Gamaliel first."
Always consider strangers as burglars, and at the same time honor them as if each of them were R. Gamaliel himself. It happened to R. Jehoshua that a man called at his house, and he gave him to eat and drink, and took him up to the roof to sleep there, and then removed the step-ladder leading to the roof. The man was a thief, and arose by night, and took things of value that Rabbi Jehoshua had on the roof and packed them up in his garment, and in the attempt to descend and carry off the booty he fell down and almost broke his neck. When R. Jehoshua came in the morning and found him in that condition, he said to him: "You ignoramus, is this the way people like you do?" He answered: "Rabbi, I did not suspect that persons of your station would remove the ladder." And he rejoined: "Did you not notice yesterday that I was cautious about you?" From that time on R. Jehoshua proclaimed that always should strangers be considered as burglars, and still one should honor them as he would R. Gamaliel.
Sources: Sacred Texts