Nachschlagewerk zu Jeschijahu 59:21

When he was nearing his end, Rav Joseph wanted assurance that the Torah would remain in his family and he fasted forty days until he saw in a dream the verse “my words which I have placed in thy mouth will not pass from out of thy mouth” (Isaiah 59, 21). Not being satisfied with this he fasted forty more days until he was shown the verse “they shall not pass from thy mouth and from the mouth of thy seed.” When he fasted another hundred days he was shown the verse “they shall not pass from thy mouth and from the mouth of thy seed and from the mouth of the seed of thy seed.” With this he was satisfied, for knowing that his grandchildren would be scholars also, he believed that the Torah would remain in his family.¹⁷⁾בבא מציעא פ״ה א׳.