“Probe with bayonets. If you encounter steel, withdraw. If you encounter mush, continue” is credited to Soviet leader Vladimir Lenin (1870-1924). THe book The Unknown Lenin:
From the Secret Archive (1996) includes this from September 1920:
 
”[W]e said among ourselves that we must probe with bayonets whether the social revolution of the proletariat in Poland had ripened.”
 
Political newspaper columnist Joseph Alsop wrote in 1975, “They (Soviets—ed.) merely follow Lenin’s advice to probe with bayonets any situation that looks mushy, withdrawing only when the bayonets meet steel.” The Lenin saying has been cited by American political figures such as Richard Nixon (1913-1994) and Grover Norquist.
 
 
Wikipedia: Vladimir Lenin
Vladimir Ilyich Ulyanov (Russian: Влади́мир Ильи́ч Улья́нов; IPA: [vlɐˈdʲimʲɪr ɪˈlʲitɕ ʊˈlʲanəf], alias Lenin (/ˈlɛnɪn/; Russian: Ле́нин; IPA: [ˈlʲenʲɪn]) (22 April [O.S. 10 April] 1870 – 21 January 1924) was a Russian communist revolutionary, politician and political theorist. He served as head of government of the Russian Soviet Federative Socialist Republic from 1917, and of the Soviet Union from 1922 until his death. Under his administration, the Russian Empire was dissolved and replaced by the Soviet Union, a one-party socialist state; all land, natural resources, and industry were confiscated and nationalized. Ideologically a Marxist, his political theories are known as Leninism.
 
16 Deember 1942, The Stars and Stripes (London), “Water a Big Problem to Both Armies” (AP), pg. 1, col. 5:
Probe With Bayonets
(...)
A bayonet stab on a mine does not explode it as the mines are set for weight of 350 pounds and over. They are aimed primarily at lorries and motor vehicles.
 
18 May 1975, Sunday Advocate (Baton Rouge,LA), “Post-Vietnam Assessment Is Intense and Painful” by Joseph Alsop, pg. 2-B, col. 2:
The Soviets have no grand, Hitlerian plan of conquest. They do not take Hitlerian risks. They merely follow Lenin’s advice to probe with bayonets any situation that looks mushy, withdrawing only when the bayonets meet steel.
 
Google Books
What They Said in 1977:
The Yearbook of Spoken Opinion
By Alan F. Pater and Jason R. Pater
Beverly Hills, CA: Monitor Book Co.
1978
Pg. 116:
[Lenin’s doctrine was]: “Probe with bayonets; if you encounter mush, proceed; if you encounter steel, withdraw”
 
Google Books
The Real War
By Richard Nixon
New York, NY: Simon & Schuster
1984
Pg. ?:
The basic rule of Soviet behavior was laid down years ago by Lenin: Probe with bayonets. If you encounter steel, withdraw. If you encounter mush, continue.
 
Google Books
The Unknown Lenin:
From the Secret Archive
By Vladimir Ilʹich Lenin
Edited by Richard Pipes
New Haven, CT: Yale University Press
1996
Pg. 98 (September 1920):
We formulated it not in an official resolution recorded in the minutes of the Central Committee and representing the law for the party and the new congress, but we said among ourselves that we must probe with bayonets whether the social revolution of the proletariat in Poland had ripened.
   
Google Groups: alt.politics.liberalism
The Politics of Bullying
Gandalf Grey
8/23/04
http://www.tompaine.com/articles/the_politics_of_bullying.php
The Politics Of Bullying
Paul Rogat Loeb
August 20, 2004
(...)
Yet Democrats and honorable Republicans should have had the courage to say that this definition was unacceptable. Instead they capitulated to the tactics of Republican strategists like Grover Norquist, who proudly quotes Lenin’s motto, “Probe with bayonets, looking for weakness.”
 
Brendan Nyhan
July 31, 2005
Grover Norquist’s Communist obsession
Grover Norquist, the head of Americans for Tax Reform, has a disturbing obsession with Communist metaphors and tactics. According to David Brock, Norquist keeps a portrait of Lenin and frequently quotes Lenin’s saying “Probe with bayonets, looking for weaknesses.”
   
Yahoo! News
August 6, 2015
Republican Presidential Debate - Live Report
AFP By Stephanie Griffith in Washington and Ivan Couronne in Cleveland, Ohio
Washington (AFP) - 03:21 GMT - AFP IS NOW CLOSING THIS LIVE REPORT on the first Republican presidential debate, the first formal campaign event ahead of the November 2016 White House election.
(...)
02:48 GMT - Look out, Putin! - “Putin believes in the old adage, you probe with bayonets. When you find steel you stop. Under Obama and Clinton we found a lot of mush,” Scott Walker said, adding that he would send weapons to Ukraine and put NATO troops on Poland’s eastern border.