Question: If a person has a strong presumption that his brother or friend is content for him to eat from the fruit of his orchard, or to eat from his food, or to take from his water, or to wear some of his clothes, and the like—may he act upon that presumption?
Answer—and Allah is the One who grants success and aid: That is permissible when there is a presumption of consent. Such a presumption arises when the orchard owner (and the like) is known by habit to overlook and be content with people eating some of his fruit, taking from his water or food, and the like.
Custom may be one of lenience and consent for relatives and friends but not for others; it may apply to a certain amount but not more; it may apply to some kinds of fruit but not others. The established custom regarding any of these matters is a way to forming a sound presumption of the owner’s consent. If there is no particular custom about the person nor any general custom in the town, one should not proceed.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2