There are two type of description about her (mother of shia mehdi):
1. Princess (may be white)
2. Black slave
Here is the infamous story of the Narjis, mother of shia Imaam Mahdi being a Roman princess and being sold into slavery in order to marry 11th Imaam. Her name was Narjis, but the LONG story of her being a Roman princess and being sold into slavery is mawdoo (Fabricated).
Here are the sources to this hadeeth:
1. Al-Sadooq, Kamaal Al-Deen, vol. 2, ch. 41, pg. 417 – 423, hadeeth # 1
2. Al-Toosi, Ghaybah Al-Toosi, pg. 208
3. Al-Majlisi, Bihaar Al-Anwaar, vol. 51, ch. 1, pg. 6 – 10, hadeeth # 12
Here is the sanad (chain of narrators) to the story. (taken from Bihaar Al-Anwaar):
غط، [الغيبة للشيخ الطوسي] جَمَاعَةٌ عَنْ أَبِي الْمُفَضَّلِ الشَّيْبَانِيِّ عَنْ مُحَمَّدِ بْنِ بَحْرِ بْنِ سَهْلٍ الشَّيْبَانِيِّ قَالَ قَالَ بِشْرُ بْنُ سُلَيْمَانَ النَّخَّاسُ وَ هُوَ مِنْ وُلْدِ أَبِي أَيُّوبَ الْأَنْصَارِيِّ أَحَدُ مَوَالِي أَبِي الْحَسَنِ وَ أَبِي مُحَمَّدٍ وَ جَارُهُمَا بِسُرَّمَنْرَأَى
- Abee / Aboo Al-MufaDDal Al-Shaybaanee (Muhammad bin ‘Abd Allaah Aboo Al-MufaDDal) is Da’eef (Weak) according to:
1. Al-Najaashee, Rijaal, pg. 396
2. Al-Hillee, Al-KhulaaSah, pg. 256 & pg. 252
3. Al-GhaDaa-iree, Kitaab Al-Du’afaa, pg. 99
4. Al-Toosi, Fihrist, pg. 402
5. Al-Toosi, Rijaal, pg. 447
6. Al-Khoei, Mu’jam Rijaal Al-Hadeeth, vol. 16, pg. 444, person # 11115
- Muhammad bin BaHr bin Sahl Al-Shaybaanee is Ghullah (exaggerator) & MufawwiDa according to:
1. Al-Toosi, Rijaal, pg. 447
2. Al-Hillee, Al-KhulaaSah, pg. 254
3. Al-Toosi, Fihrist, pg. 390
4. Al-Najaashee, Rijaal, pg. 384
5. Al-GhaDaa-iree, Kitaab Al-Du’afaa, pg. 98
6. Al-Khoei, Mu’jam Rijaal Al-Hadeeth, vol. 15, pg. 122, person # 10297
- Bishr bin Sulaymaan Al-Nakhaas is Majhool (Unknown) according to:
1. Al-Khoei, Mu’jam Rijaal Al-Hadeeth, vol. 3, pg. 316, person # 1755
Based on this sanad (chain of narrators), this hadeeth is graded as Mawdoo (Fabricated) or at least a minimum of Da’eef Jiddan (VERY WEAK).
Also, here is what Al-Khoei has said regarding this hadeeth / story under Bishr bin Sulaymaan.
لكن في سند الرواية عدة مجاهيل، على أنك قد عرفت فيما تقدم أنه لا يمكن إثبات وثاقة شخص برواية نفسه.
“But the sanad (chain of narrators) in the hadeeth / story has many majhool (unknown) people, and you know that it one cannot establish his own trustworthiness through his own narration about himself”
Source: 1. Al-Khoei, Mu’jam Rijaal Al-Hadeeth, vol. 3, pg. 316, person # 1755
One of the reasons for this fabricated story could be to make the mother of shia 12th Imaam come from a noble lineage as his father (Imaam Al-‘Askaree) is already from a noble lineage. So noble+noble becomes super power noble (can raise people from dead etc)!
Al-Kulaynee has added a tradition in Al-Kaafi that alludes to the mother of shia Imaam Mahdee being from Al-Nawbah, which is in Sudan, but this hadeeth is mursal (disconnected) and cannot be used as daleel (proof) (See: Al-Kaafi, vol. 5, pg. 352, hadeeth # 2).
There have been other Haadeeth that shows the mother of Imaam Mahdee (AS) being a black slave, but the authenticity of these reports has not been determined.
So finally i say: Why shia don’t ask this to his hidden imam! Or may be this is the solid reason of his ghayba!