Ahmed, a former MP from Bihar, said that Nitish Kumar was instrumental in breaking off with the Bharatiya Janata Party last year after it decided to elevate Modi as head of its national campaign for Lok Sabha polls.
Modi would be sworn in as prime minister in a few days and Nitish Kumar apparently did not want to work with him, he claimed.
"This is my hunch that Nitish Kumar as chief minister did not want to work with Narendra Modi as prime minister," Ahmed told IANS.
Nitish Kumar resigned Saturday, taking moral responsibility for the rout of his Janata Dal-United in the state in the general elections.
JD-U won only two of the 40 Lok Sabha seats from Bihar. In 2009, it won 20 seats in alliance with the BJP.