Reference for Bekhorot 111:22

הלקוח או שניתן לו במתנה פטור מן מעשר בהמה

Said R'Eleazar: R'Johanan appeared [last night] to me in a dream [therefore I know] that I will say a good thing [to-day], [as follows]: Scripture says: 'Thou shalt do', intimating that only where the act of consecratio is possible does Scripture impose restrictions.

Jastrow