Talmud - Mas. Shabbath 31a [Our Rabbis taught: A man should always be gentle like Hillel, and not impatient like Shammai.]

It once happened that two men made a wager with each other, saying, He who goes and makes Hillel angry shall receive four hundred zuz. Said one, I will go and incense him.
That day was the Sabbath eve, and Hillel was washing his head. He went, passed by the door of his house, and called out, "Is Hillel here, is Hillel here?"
Thereupon he robed and went out to him, saying, "My son, what do you require?"
"I have a question to ask", said he.
"Ask, my son", he prompted.
Thereupon he asked: "Why are the heads of the Babylonians round?"
"My son, you have asked a great question", replied he: "because they have no skillful midwives."
He departed, tarried a while, returned, and called out, "Is Hillel here; is Hillel here?"
He robed and went out to him, saying, "My son, what do you require?"
"I have a question to ask", said he.
"Ask, my son", he prompted.
Thereupon he asked: "Why are the eyes of the Palmyreans bleared?"
"My son, you have asked a great question", replied he: "because they live in sandy places."
He departed, tarried a while, returned, and called out, "Is Hillel here; is Hillel here?"
He robed and went out to him, saying, "My son, what do you require"
"I have a question to ask", said he.
"Ask, my son", he prompted.
He asked, "Why are the feet of the Africans wide?"
"My son, you have asked a great question", said he: "because they live in watery marshes."
"I have many questions to ask", said he, "but fear that you may become angry."
Thereupon he robed, sat before him and said, "Ask all the questions you have to ask".
"Are you the Hillel who is called the nasi of Israel?"
"Yes", he replied.
"If that is you", he retorted, "may there not be many like you in Israel."
"Why, my son", queried he.
"Because I have lost four hundred zuz through you", complained he. "Be careful of your moods", he answered, "Hillel is worth it that you should lose four hundred zuz and yet another four hundred zuz through him, yet Hillel shall not lose his temper."