This comes after Vice President Hamid Ansari gave his approval to the CBI to prosecute Mr Sahni.
The party asked him to explain his conduct before former party President Sharad Yadav, who is leader of the party in the Rajya Sabha within a week.
"A chargesheet has been filed against you into the LTC case. It has become a serious concern after Chairman of Rajya Sabha (Hamid Ansari) has granted permission for judicial proceedings into the matter. It is very clear to you that such involvement in financial irregularities are against pronounced policies of the party.
"Hope you will explain your position before leader of JD-U in Rajya Sabha Sharad Yadav within a week. In case you fail to do so, the party can take disciplinary action against you," JD-U general secretary KC Tyagi said in the showcause notice sent to Mr Sahani.
"I have not received any showcause notice. I am in Muzaffarpur and the notice might have been delivered in Delhi (residence)...But I'll explain my position in my reply to the party," Mr Sahni told Press Trust of India.
"It's a conspiracy against me... Why shall I resign on moral ground when I have not done anything wrong," the second term Rajya Sabha member from Nitish Kumar's party had said in Patna yesterday.
Mr Ansari had a few days back given his approval to CBI to prosecute Sahni against whom the agency had filed a charge sheet for alleged cheating and corruption in LTC Scam.
Usually it is the party president to whom the lawmakers are asked to explain their conduct in such circumstances. However, the party has been maintaining that Nitish Kumar's election as party president replacing Sharad Yadav does not mean that the latter has "retired" and that Yadav would continue to play a key role in party matters.
The CBI had filed a charge sheet against Mr Sahni alleging that he, in conspiracy with other persons, used forged e-tickets and fake boarding passes to defraud Rajya Sabha to the tune of Rs 23.71 lakh as travel and dearness allowance reimbursement without undertaking the actual journey.
Besides Mr Sahni, whose term in the Rajya Sabha ends in April 2018, others named in the CBI chargesheet were Anup Singh Panwar, an employee of Delhi-based Air Cruise Travels Private Limited, NS Nair, then Office Superintendent (Traffic), Air India and one Arvind Tiwari.
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