The investment firm of William Ackman recently bought 146.5 million shares of Citigroup, the activist investor told CNBC Wednesday without detailing the reason behind the purchase.

Citigroup Center

At the end of his speech at the 15th annual Ira Sohn investment research conference, Ackman told reporters, "And by the way, we bought about 150 million shares of Citigroup, but I don't have time to talk about it."