Reference for Kiddushin 39:15
שלא תהא אתה אוכל פת נקיה והוא אוכל פת קיבר אתה שותה יין ישן והוא שותה יין חדש אתה ישן על גבי מוכים והוא ישן על גבי התבן מכאן אמרו כל הקונה עבד עברי כקונה אדון לעצמו
Now, I know this only of a slave sold to a heathen: since he may be redeemed by his kinsmen, his [the master's] hand is nethermost.<span class="x" onmousemove="('comment',' I.e., he is at a disadvantage, the lower value always being the basis for redemption.');"><sup>18</sup></span> How do we know it of one who is sold to a Israelite! - Because sakir [an hired servant] is stated twice, for the purpose of a gezerah shawah.<span class="x" onmousemove="('comment',' A slave sold to a Jew: as an hired servant (sakir) . . he shall be with thee - ibid. 40; a slave sold to a heathen: according to the time of an hired servant (sakir) he shall be with him - Ibid. 50. The same word used in both sections denotes that the same law applies to both.');"><sup>19</sup></span>