Answer
(Fatwa: 468/468/M=1431)
It is not known from where Molvi sahib told this. It has no reality in Islamic religion. Ansari and Sayyed can marry each other. In the question mentioned above if your parents and the parents of girl are agreed and between both of you (boy and girl) there is no reason which makes nikah unlawful i.e. fosterage (raza?at) etc, then both of you can marry each other.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband