Reference for Bava Batra 125:6

אמאי אין אדם מקנה דבר שלא בא לעולם כיון דאמר ליה על מנת שמעשר ראשון שלי שיורי שייריה למקום מעשר

Our Rabbis taught: If a Levite sells a field to any [ordinary] Israelite<span class="x" onmousemove="('comment',' I.e., one who is neither priest nor Levite, ');">¹⁰</span> with the stipulation that the first tithe<span class="x" onmousemove="('comment',' According to the Rabbinical interpretation of Deut. XIV, 22-29, three tithes had to be taken from agricultural produce, the 'first' which had to be given to the Levite, the 'second' which had to be eaten in Jerusalem, and the 'third' which had to be given (once in three years) to the poor. ');">¹¹</span>

Jastrow