Source, in full
As quoted in To Rise from Earth (1996) by Wayne Lee; some websites quote him as saying "I looked and looked and looked but I didn't see God." on 14 April 1961, a couple days after his historic flight, but the authenticity of such statements have been disputed; Colonel Valentin Petrov stated in 2006 that the cosmonaut never said such words, and that the quote originated from Nikita Khrushchev's speech at the plenum of the Central Committee of the CPSU about the state's anti-religion campaign, saying "Gagarin flew into space, but didn't see any god there." Gagarin himself was a member of the Russian Orthodox Church.
Notes from Wikiquote
As quoted in What's Missing Inside You? (2006) by Paul Schlieker, p. 17
Variants
No I didn't see God. I looked and looked but I didn't see God.