Summary:
Hadith al-Silsilah al-Dhahab (Hadith of the Golden Chain) is a hadith narrated from Ali al-Ridha, the eighth Imam of the Shia. The "chain" is a reference to the continuity of spiritual authority which is passed down from Muhammad to Ali ibn Abi Talib, through each of the Imams, to Imam Ridha. As transmitters of Hadith, the Imams link subsequent generations to the teachings of Mohammad.
Wherever he stopped, during his journey towards Marv, he caused for many blessings upon the people. At one point on his way, he entered the city of Neishaboor. A large crowd of people gathered to welcome his entrance. Meanwhile two learned men who had recorded many prophetic traditions came with scholars of Hadith, took the halter, and said:
"You, the great Imam and you, the son of great Imams, for the sake of your innocent fathers and your great grandfathers, show us your shining face and narrate us a tradition from your fathers and your great grandfather, the messenger of Allah, to be as a reminder for us".
Imam ordered to stop and the people obeyed. Once they saw his face, they became so happy that some of them start crying and some who were close to him kissed his carriage. A big clamor was made throughout the city so, the elders of the city wanted people to be quiet so they could hear a tradition from Imam Redha(A.S.).
After a while the crowd got silent and Imam dictated this tradition word by word from his honored father which was narrated from his pure grandfathers and from the messenger of Allah (S.A.W.) and from Gabriel and from Almighty Allah that:
"The word of La Ilaha Illa al-Allah is my fortress (then whoever says this has entered my fortress) and whoever enters my fortress will be saved from my punishment". Imam paused for a while and then said, "But on some conditions and I am one of its conditions."
Hadith al-Silsilah al-Dhahab (Hadith of the Golden Chain) is a hadith narrated from Ali al-Ridha, the eighth Imam of the Shia. The "chain" is a reference to the continuity of spiritual authority which is passed down from Muhammad to Ali ibn Abi Talib, through each of the Imams, to Imam Ridha. As transmitters of Hadith, the Imams link subsequent generations to the teachings of Mohammad. This transmission makes the Hadith of the Golden Chain valued as among the most truthful and accurate of all Hadiths for the Shi'ite.
The hadith is important to Shia because it implies that, on one hand, monotheism takes the believers to "Allah's fortress", which is a safe shelter, and from the other hand Imamah is a fundamental precondition of entering this shelter.
Note: "لا إلهَ إلَّا اللّهُ حِصنی، فَمَن دَخَلَ حِصنی أمِنَ مِن عَذابی. قالَ: فَلَمّا مَرَّتِ الرّاحِلَةُ نادانا: بِشُروطِها، و أنَا مِن شُروطِها" Oyoun Akhbar Al-Ridha, v, 2 p, 135
Source:http://www.imamreza.net/eng/imamreza.php?id=8512