四分律比丘含注戒本 2.145

2.145

謂隨轉側，脇著床，結墮。不犯者：先不知；若語已住，先與開間；若間寬廣，不相妨閡；若親舊人教言「但敷，我自語主」；若倒地；若病，轉側墮上；若力勢所持；若被繫閉；命、梵二難——並不犯。

(This means that each time one turns over or one's flank touches the couch, a prāyaścittika is incurred. Regarding non-offense: if one previously did not know; if one stayed after having spoken [to him] and [the other] previously made space for him; if the space is wide and does not obstruct; if a relative or acquaintance instructed him, saying, “Just spread it, I will speak to the owner myself”; if one falls to the ground; if one is sick, turns over, and falls upon it; if one is restrained by physical force; if one is locked up; or due to the two perils of life and chastity—none of these constitute an offense.)