According to multiple sources, including a book written by Gagarin himself, before the lift-off, the Chief Designer, Sergei Pavlovich Korolev, tried to cheer Gagarin up by telling him:

• "Now, off you go! Do a good job!"

This was said in a simple and supportive manner, aiming to encourage and reassure Gagarin before his historic flight.

Sources:

1. "Road to the Stars" by Yuri Gagarin