The JD-U, however, demanded that Tejaswi should come out 'clean'.
Bihar's ruling coalition partner's assertion came after Rashtriya Janata Dal (RJD) chief Lalu Prasad Yadav dismissed the resignation of his son Tejaswi Yadav.
"Nitish Kumar is a person of high moral values. Whatever he had to say has been said during the Vidhan Mandal meeting earlier. Nitishji did not demand a resignation. However, to continue to uphold the principles of the party, it is necessary that the concerned minister comes out clean," JD (U) leader KC Tyagi told ANI here.
In response to Lalu's earlier remark of an FIR being insufficient to resign, Tyagi said an FIR is lodged only on the basis of a thorough preliminary investigation.
He further urged the necessity to bring the truth to the forefront.
Amid reports of an apparent rift between the parties, Tyagi called the alliance their baby and said nothing will be done to harm it.
"The JD (U) and Nitish Kumar are the last people who will do anything to tamper with the mahagathbandhan. It is our baby, which was initiated by Nitish Kumar. There is no question of ending it at all," he said.
Speaking exclusively to ANI, Lalu had earlier stated that the grand alliance in Bihar is an unbreakable bond, adding that the RJD will not let the Bharatiya Janata Party (BJP) or the Rashtriya Swayamsevak Sangh (RSS) break the alliance between the JD (U), Congress and the RJD.
However, there have speculations of a split in the coalition after the JD (U) urged Tejaswi Yadav to come out clean on the corruption charges imposed by the CBI and the ED and asked Lalu to declare his assets and reveal the source of income.
Earlier last week, the Central Bureau of Investigation (CBI) registered a corruption case against Lalu, his wife Rabri Devi, son Tejaswi; former Indian Railway Catering and Tourism Corporation (IRCTC) Managing Director P K Goyal; and the wife of Lalu's confidante Prem Chand Gupta, Sujata on allegations of awarding the tender for development, maintenance and operation of hotels in Ranchi and Puri in 2006.
The CBI later questioned Rabri Devi and Tejaswi.
However, Lalu on Friday said that his son Tejaswi Yadav, who is facing corruption charges, will not resign as Bihar Deputy Chief Minister.
"The Vidhan Mandal of our party has taken a decision that Tejaswi Yadav will not resign," Lalu told ANI in an exclusive conversation.
"An FIR is not sufficient reason for his resignation," he added.
This assertion from the RJD chief thus brought an end to speculations that Tejaswi Yadav might step down from the post to save the ruling alliance.