Hazrat Mansoor and Hasan state that it is permissible to fast in a state of janabah (major ritual impurity) for a voluntary (nafl) fast, but for an obligatory (fard) fast, its qada (make-up) must be performed.
Hadith Referenceمصنف ابن ابي شيبه / كتاب الصيام / 9834
Hadith Takhrij(مصنف ابن ابي شيبه: ترقيم سعد الشثري 9834، ترقيم محمد عوامة 9672)