Musnad Ahmad 31:60

Mu'ammal ibn Isma'il narrated to us, he said: Hammad—meaning ibn Salamah—narrated to us, al-Jurayri narrated to us, from Abu Nadrah, from Abu Sa'id: That the Messenger of Allah (peace and blessings of Allah be upon him) said: "If one of you comes to a garden and wishes to eat, let him call out: 'O owner of the garden!' three times. If he answers him, then [he should ask permission], and if not, then let him eat. And if one of you passes by camels and wishes to drink from their milk, let him call out: 'O owner of the camels!'—or 'O shepherd of the camels!'—if he answers him, then [he should ask permission], and if not, then let him drink. And hospitality is three days; whatever is beyond that is charity."