Uber shares fell 4.6% at $44 in extended trading before erasing most of its losses.
Any buyer will have a 30-day lockup period, the source said.
The person added that the Japanese investment giant's motive was unrelated to the performance of Didi Global (DIDI.N) and Alibaba and SoftBank just thought it was a good time to cash out partially on its Uber stake and take some profit.
CNBC had first reported the sale, saying it was to cover for losses seen in its investment in Didi and Alibaba.
So far this month, Didi has fallen 37% while Alibaba has lost 14%.
SoftBank's stake in Uber is now down to less than 100 million shares, CNBC added, citing one source.
Following the news, SoftBank Group shares rose 4.1%.
"Gains from selling Uber shares should help to offset losses on other shares held in its portfolio to an extent," said Jun Kitazawa, strategist at Miki Securities.
SoftBank shares have been under pressure in recent weeks on an unrelenting stream of crackdowns out of Beijing on fast-growing Chinese firms, ranging from Didi to edtech firms.
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