Musnad Abu Ya'la al-Mawsili 42:271

Zuhayr narrated to us, Yazid bin Harun narrated to us, al-Jurayri informed us, from Abu Nadra, from Abu Sa’id al-Khudri, from the Prophet (peace be upon him), who said: "If one of you comes upon a shepherd, let him call out, 'O shepherd of the camels!' three times. 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 three days, and whatever is beyond that is charity."