Reference for Bava Kamma 139:5

ואמרי נהרדעי אורכתא דלא כתיב ביה זיל דון וזכי ואפיק לנפשך לית ביה מששא מ"ט משום דא"ל האיך לאו בעל דברים דידי את

whereas where it is not denied, we would be able to execute. The Nehardeans further said: An assignment which does not contain the words, 'Go forth and take legal action so that you may acquire title to it and secure the claim for yourself' is of no validity, the reason being that the defendant might say to him:<span class="x" onmousemove="('comment',' The assignee. ');">⁴</span>

Jastrow