Manava Dharmashastra 9.168

9.168

mātā pitā vā dadyātāṃ yam adbhiḥ putram āpadi
sadṛśaṃ prītisaṃyuktaṃ sa jñeyo dattrimaḥ sutaḥ //

That (boy) equal (by caste) whom his mother or his father affectionately give, (confirming the gift) with (a libation of) water, in times of distress (to a man) as his son, must be considered as an adopted son (Datrima).