Reference: Fataawa ash-Shaykh al-‘Allaamah Muhammad Naasiruddeen al-Albaanee fil-Madeenah wal-Imaaraat – Page 123
Fataawa al-Madeenah – Page 50
Question: Is the hajj of the woman [who travelled] without her mahram nullified?
Response: [No], it is not nullified, rather, she has sinned [by travelling for hajj without her mahram].