I came across this saying (hadith?) about Salat Al layl:

One who does not read the Salat al-Layl is not considered as a Shi`a of the Ahlul Bayt.

I searched and found that the source comes from:

Bihar Al-Anwar, Vol. 87, Pg. 162; Maqnah of Shaikh Mufid, Pg. 111

I am still questioning this quote although it is from a special book as Bihar Alanwar. I want to read what is written before and after that quote and why Salat Allayl itself allow a person relate to being Shia.
Sadly, I couldn't find the whole page which the quote were taken from. I would be so thankful if you could help me.

I have been taught that Wilayat of Imam Ali as is enough. If a person have the wilayat and also act upon the obligations but doest not pray Salat Al layl, isn't she considered as Shia? This is what confuses me with this quote. I understand that Salat Al layl has high reward, but after all it is not obligatory to perform. How come that consideration of being Shia depends on an Mustahabb act?

The tradition you quoted on salat al-Layl is authentic.
The meaning of the tradition is deeper. The word shia here doesn’t mean ordinary Shias. It means the first ranking Shias. Such as the position obtained by Maitham al-Tammar, Habib ibn mazhar etc.
Shias are different level. The lowest level is to believe Imam Ali and Ahlul bait are the legitimate successors of the prophet.