Reference for Chagigah 6:9

אמר לו לך אמור להם אל תחושו למניינכם כך מקובלני מרבן יוחנן בן זכאי ששמע מרבו ורבו מרבו הלכתא למשה מסיני עמון ומואב מעשרין מעשר עני בשביעית מה טעם הרבה כרכים כבשו עולי מצרים ולא כבשום עולי בבל

[R. Eliezer] said to him, Go and tell them, Do not be concerned about your tally, I have received a tradition from R. Yochanan ben Zakai, who heard from his teacher, and his teacher from his teacher until it was a law transmitted to Moses at Sinai, that Ammon and Moab must give the poor man’s tithe in the Sabbatical year. What is the reason? Many cities were conquered by those who came up from Egypt, which were not conquered by those who came up from Babylon;

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