Midrash su Ecclesiaste 11:6

(Ib. b) We are taught that R. Joshua says: "Although a man took a wife while he was young, he should nevertheless marry again [if he remains alone] when he becomes old; although one has children from his early years, he should nevertheless try to have children even at advanced age, as it is said (Ecc. 11, 6) In the morning (early years) sow thy seed, and in the evening let not thy hand rest; for thou knowest not which will succeed, whether this or that, or whether both of them will be alike good." R. Akiba says: "Even if one studied the Torah in his youth, he should nevertheless continue to study it when in his advanced age; although one had disciples in his early years, he should nevertheless acquire disciples in his advanced age, as it is said (Ib.) In the morning," etc. It was said that R. Akiba had twelve thousand pairs of disciples who came from all over the country between Gabbath and Antipatris, and all of them died in one season because [of the sin that] they did not treat each other with sufficient respect; and the [Jewish] world was demoralized until R. Akiba came to our southern Rabbis and taught unto R. Maier, R. Juda, R. Jossi, R. Simon and R. Elazar b. Shamua, and it was they who preserved the Torah in that [grave] hour. We are taught [in a Baraitha] that all the disciples died between Passover and the Feast of Weeks. R. Chana b. Abba, and others say R. Chiya b. Abin, said: "All died a horrible death." What was it? Said R. Nachman: "It was the croup."

And Abraham took another wife. Scripture says in reference to this verse: In the morning sow thy seed; and in the evening withhold not thy hand (Eccles. 11:6).

R. Akiba said: In the morning sow thy seed (Eccles. 11:6) means that if you have trained disciples in your youth, you should not cease to do so in your old age. It is related that R. Akiba had three hundred disciples during his earlier years, but that all of them died. Had he not raised seven disciples in his old age, no one would have survived to speak in his name. Another explanation of In the morning, sow thy seed: R. Yosé held that this means: If you marry in your youth and your wife bears a child and then dies, you must remarry in your later years, for you have no way of knowing which offspring will be the worthiest. From whom do you learn this? From Abraham, for during his earlier years he had only two sons, but in his old age he had twelve more.