Sayyed Mohammad Al-Musawi,
Sayyed Mohammad al-Musawi is originally from Iraq and heads up the World Ahlul Bayt Islamic League in London. Other than being involved in various humanitarian projects, he frequently responds to religious questions. In the past, he has also spent significant time in India guiding the community.
The Hadeeth stating that who dies without allegiance to the Imam of his time, will die the death of Jahiliyyah (Ignorance) is a Mutawaatir Hadeeth which means narrated by many authentic narrators that can never jointly lie.
It has been narrated in many Sunni references like:
1. Musnad Ahmad Ibn Hanbal, V. 28, P. 88.
2. Saheeh Muslim, V. 3, P. 1478.
3. Hilyatul Awliyaa' by al-Asbahani, V.3, P. 244.
4. Kitab Al-Sunnah by Ibn Abi 'Aasim , V.2, P.503.
5. Al-Sunan Al-Khubra by al-Bayhaqi, V.8, P. 270.
6. Saheeh ibn Hibbaan, V.10, P.434.
7. Sunan Abi Dawood, V.3, P.425.
8. Al-Mo'jam Al-Kabeer by al-Tabaraani, V.19, P.334. And many other Sunni books.
Main Shia books as well narrated this Hadeeth like:
Kitab Al-Kaafi by al-Kulaini 2; 21,
Kamaal Al-Deen by Al-Sadouq;2;409,
and Kifayat Al-Athar by Al-Khazzaz 296.