What is the correct way to refer to the location of a part of a Quranic verse when writing it: is it (number: surah) or (from verse number: surah)?

It is permissible to mention part of a verse without indicating its location or stating that it is a verse from such and such a surah. This is evidenced by the Prophet, peace and blessings be upon him, and his successors quoting some verses without mentioning their source. If a person mentions a verse and wishes to refer to its location within its surah, there is no harm in that. It may even be preferable to mention that what was quoted is part of a specific verse, because it is clearer. So, there is flexibility in this matter.