You aren’t learning anything when you’re talking.

Lyndon B. Johnson (1908–1973) American politician, educator, US President (1963–69)

(Attributed)
    (Source)

Sometimes given as "You ain't learning anything when you're talking."

Reported, not as a quote, but as a sign on his wall while a US Senator, in Leslie Carpenter, "A Man of Complexity," Boston Herald (1963-12-01), read into the Congressional Record, House of Representatives (1963-12-03) by House Speaker John W. McCormack (D-RI).