The Reason for Affiliation to Imam Zayd ibn ʿAlī(Peace be Upon Them Both) and Its Meaning:

When misguidances appeared, darkness spread, inclinations became divided, and opinions became scattered in the days of the Umayyad Caliphate
even though disagreement had emerged in this Ummah after the death of the Messenger (May Allah bless him and his family and grant them peace), yet tribulations grew great and calamities became immense in this state—and one who was not from the people of Islam became clothed with Islam, and one who does not even hover around it claimed it, and Imam Zayd ibn ʿAlī (Peace be Upon Him) rose to wash the religion clean and renew what the Messenger of the Lord of the worlds (May Allah bless him and his family and grant them peace) brought, leading a group from his Household and their allies; and it is the group that Allah promised the Ummah, upon the tongue of its Prophet (May Allah bless him and his family and grant them peace), that it will not cease to be manifest upon the truth, fighting for it until the Day of judgment. The Ahl al-Bayt (Peace be Upon Them) announced their affiliation to Imam Zayd ibn ʿAlī (Peace be Upon Him)—meaning that they worship Allah by what he worships Allah by: from tawh ī d, justice, and imamate—so that they might show the servants what they call them to of Allah’s upright religion and His straight path. And he had established the proof and made the path clear after his forefathers (May Allah’s blessings be upon them), so they chose him as a banner between them and the Ummah of their grandfather.
Imam al-Ka mil ʿAbdulla h ibn al-H asan ibn al-H asan (Peace be Upon Him) said: “The banner between us and the people is ʿAlī ibn Abī T a lib (Peace be Upon Him), and the banner between us and the Shī ʿa is Zayd ibn ʿAlī (Peace be Upon Him).”
And his son, Imam Muh ammad ibn ʿAbdulla h al-Nafs al-Zakiyyah (Peace be Upon Him), said: “Indeed, by Allah, Zayd ibn ʿAlī (Peace be Upon Him) revived what had been effaced of the Sunnah of the messengers, and he set upright the pillar of the religion when it had become crooked; and we shall not take except from his light, and Zayd is the Imam of the Imams”—end.
So the supplication of the Imams has not ceased, and it remains upon that, Inshallah, until the Day of Resurrection.
And Abu H anī fah al-Nuʿma n ibn Tha bit—who died in the year one hundred and fifty—was among the students of Imam Zayd ibn ʿAlī (Peace be Upon Him) and his followers.
[Al-Tuh af: Sharh al-Zulf, by the Imam who renewed the religion, Majd al-Dī n ibn Muh ammad ibn Mans u r al-Muʾayyidī (Peace be Upon Him).]