Reference for Megillah 55:19

בשעת פטירתו א"ל [רבי] ברכני א"ל יהי רצון שתגיע לחצי ימי ולכולהו לא אמר לו הבאים אחריך בהמה ירעו

When he was about to die, Rabbi said to him: Bless me. He said to him: May it be God’s will that you live to half my days. Not to their whole length? He said back: Those who come after you, shall they graze cattle?

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