Midrash su I Re 12:7

(Fol. 10) Our Rabbis were taught concerning the passage (Lev. 4, 22) When a ruler sinneth. This excludes [from being a ruler] one who was sick with leprosy, as it is said (II Kings 15, 5) And the Lord smote the King so that he was a leper unto the day of his death, and he dwelt in a house of freedom. Since the passage says in the house of freedom, we understand that until then (until stricken) he was a slave (a king); as it happened when Rabban Gamaliel and R. Joshua, who were on a boat; the former had taken bread for food, and the latter had taken bread and fine flour for food. When the bread of Rabban Gamaliel was consumed by him he relied upon the fine flour of R. Joshua. Upon R. Gamaliel's question: "Were you then aware that there will be a delay in the journey that you took along the fine flour in reserve?" R. Joshua replied: "There is a star which appears once in seventy years that makes the captains of the ships err, and I thought perhaps it would appear now and make us err [I therefore took along extra food]." "You possess so much wisdom!" exclaimed Rabban Gamaliel, "and yet you are compelled to go on a ship to make your living." To which R. Joshua rejoined: "You are wondering about myself; how would you be surprised if you knew about two disciples of yours who are on the land, viz., R. Jochanan b. Gudgada and R. Elazar b. Chasma, who can figure out how many drops of water there are in the sea and nevertheless have no bread to eat and no garments to wear." Rabban Gamaliel then made up his mind to appoint them as officers. Upon his return he sent for them, but they did not appear; he sent again for them, and when they came he said to them: "Do you think that the appointment to which I am selecting you is lordship. (Ib. b.) Nay, slavery I give to you, as it is said (I Kin. 12, 7) And they spoke unto him saying, If thou wilt this day be a servant unto this people, etc."