Musnad Abu Ya'la al-Mawsili 42:314

Zuhayr narrated to us, Yazid bin Harun narrated to us, al-Jurayri informed us, from Abu Nadra, from Abu Sa’id, from the Prophet (peace be upon him), who said: "If one of you comes upon a camel herder, let him call out three times: 'O herder of the camels!' If he answers him, then [he may ask]; otherwise, let him milk and drink, but let him not carry [any away]. And if one of you comes upon a garden wall, let him call out three times: 'O owner of the garden!' If he answers him, then [he may ask]; otherwise, let him eat, but let him not carry [any away]." And the Messenger of Allah (peace be upon him) said: "Hospitality is for three days; whatever is beyond that is charity."