Nedarim, Chapter 4, Mishnah 4. This Mishnah deals with someone who vowed not to derive benefit from his fellow, and it spells out which forms of contact between the two are permitted and which are forbidden. To make things clear, we will use an example: Reuven vowed not to derive benefit from Shimon.
Visiting the sick:
"Hamudar hana'ah meichavero... venichnas levakro" - Reuven fell ill, and Shimon came to visit him and to fulfill the mitzvah of visiting the sick. In such a case the Mishnah says "omed aval lo yoshev" - Shimon stands during the visit and stays there for a while, but he may not sit down.
The Gemara explains the reason: this refers to a place where it was customary to pay a person to sit beside a sick man or an elderly man. If Shimon sits beside Reuven without taking a wage for it, he ends up providing Reuven with a benefit that is forbidden to him. Standing, however, is permitted, since by standing he is merely fulfilling the mitzvah of visiting the sick, and people do not normally receive payment for that.
Healing:
"Umerap'ehu refu'as nefesh" - he may heal him personally, in his body, even though benefit from him is forbidden, because healing a person is a mitzvah.
"Aval lo refu'as mamon" - he may not heal the things that belong to him and are part of his property, such as his animal.
Bathing:
"Rocheitz imo b'ambati gedolah" - he may bathe together with him in a large bath. "Aval lo biktanah" - in a small bath it is noticeable that the water level rises because of the additional person who entered it, and so this is a forbidden benefit.
Sleeping in one bed:
"Yashan imo bamittah" - Reuven may sleep in the bed, provided that it is not Shimon's own bed, for if it were, he would be deriving benefit from him. "Rabbi Yehudah omer: bimos hachamah" - in his view this is permitted only during the summer months, when it is hot and a person has no need for the other's body heat. "Aval lo bimos hatzinah" - in the winter months it is forbidden, because he thereby provides him with a benefit: the additional body heat in the bed.
Eating together:
"Meiseiv imo lamittah" - they may recline together for their meal.
"V'ochel imo lashulchan" - they may even eat together at one table.
"Aval lo min hatamchui" - they may not eat together from the same bowl, lest one set aside a choice portion for the other and thereby provide him with benefit.
"Aval ochel hu imo min hatamchui hachozer" - a bowl whose food is returned to the master of the house, since it contains such a quantity of food that leftovers always remain, and so he has no influence at all on his fellow.
Similarly, Shimon does not eat together with Reuven from the trough of food set before the workers, for Shimon, who is forbidden to provide Reuven with benefit, might leave an especially choice piece there for him.
Working in the field:
Shimon may not work together with Reuven in the same row in the field, for if Shimon hoes there, it is to Reuven's advantage. According to one opinion they may work in the same furrow provided that they keep their distance from one another; according to the opposing opinion, even at a distance it is not permitted, lest he come to do so right next to him, in a way that Reuven would receive direct benefit from him.
In summary: In this Mishnah we learned the limits of permitted contact between the one who took the vow and the one from whom he vowed not to benefit: visiting the sick while standing but not sitting; healing the body but not healing one's property; bathing in a large bath but not in a small one; sleeping in a bed that is not his fellow's (and according to Rabbi Yehudah, only during the summer months); reclining and eating at one table, but not from a shared bowl, only from the bowl that is returned; and the prohibition against working together in the same row in the field. The principle that emerges from all of them is one: wherever there is a concern of actual benefit, it is forbidden, and wherever there is nothing more than the fulfillment of a mitzvah, it is permitted.