Midrash for Kiddushin 43:4

אמר בסוף שש ולא אמר בתחילת שש אינו נרצע שנאמר אם אמר יאמר העבד עד שיאמר כשהוא עבד

but not at the end, he is not bored, for it says.' I will not go out free': [hence] he must say it when about to depart. If he says it at the end of the six[th year], but not at the beginning, he is not bored, for it is said: 'But If the slave shall plainly say': he must say it while still a s

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