Reference for Bava Batra 105:1
שלא בפניו צריך למימר ליה לך חזק וקני
but [if the act is] not [done] in his presence, he must say, Go, occupy and acquire ownership. Rab inquired: What is the rule in the case of a gift? Said Samuel: What is Abba's<span class="x" onmousemove="('comment',' Rab. v. supra p. 214, n. 9. ');"><sup>1</sup></span> difficulty? Seeing that in the case of a sale where the purchaser gives money, if the seller says to him, 'Go, occupy and acquire ownership,' he does acquire ownership but otherwise not, how much more so in the case of a gift?<span class="x" onmousemove="('comment',' I.e., a fortiori, if the recipient of the gift does not take possession in the donor's presence, the latter must use this formula to make the gift valid. ');"><sup>2</sup></span>