Fath al-Rabbani

Benefits: … In customary matters, as long as there is no prohibition, permissibility remains established. Since there is no prohibition regarding parting the hair, therefore, parting the hair is permissible, and not parting it is also permissible, because there is no command regarding parting it either. From the Prophet sallallahu alayhi wa sallam, both parting the hair and not parting it are established. The fundamental reason for this is that the Shari‘ah has not given any specific ruling regarding it; under circumstances, either of the two can be chosen. In such matters, the Prophet sallallahu alayhi wa sallam’s conformity with the People of the Book (Ahl al-Kitab) was for the purpose of winning their hearts, so that perhaps they might incline towards Islam. However, when it was felt that such conformity was not beneficial, the Prophet sallallahu alayhi wa sallam abandoned it. The Messenger of Allah sallallahu alayhi wa sallam also preferred conformity with the People of the Book because, at the very least—even if only in claim—they professed adherence to a revealed religion, whereas the polytheists (mushrikin) were outright idolaters. The hair should be parted in the middle, because the noble habit of the Messenger of Allah sallallahu alayhi wa sallam was to part the hair from the middle. And Allah knows best.

See also the chapter under hadith number (8225).