Who are the "Grandsons of the Messenger of Allah," peace and blessings be upon him?

Al-Hasan and Al-Husayn, the sons of Ali ibn Abi Talib and Fatimah, the daughter of the Messenger of Allah, may Allah bless him and grant him peace, are the grandsons of the Messenger of Allah, may Allah bless him and grant him peace. It has been authentically narrated that Al-Husayn is one of the "Asbat" (grandsons/tribes), as in the hadith: "Husayn is from me and I am from Husayn. May Allah love whoever loves Husayn. Husayn is a 'sibt' (grandson/tribe) among the 'Asbat'." Likewise, it was common among the early scholars to refer to Al-Hasan and Al-Husayn as "the two Sibṭayn" (the two grandsons).