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).