it was happy birthday to you
"On 12 April 1961, Yuri Gagarin became the first man to travel into space, launching to orbit aboard the Vostok 3KA-3 (Vostok 1). His call sign in this flight was Kedr (Cedar; Russian: Кедр). During his flight, Gagarin famously whistled the tune "The Motherland Hears, The Motherland Knows" (Russian: "Родина слышит, Родина знает"). The first two lines of the song are: "The Motherland hears, the Motherland knows/Where her son flies in the sky". This patriotic song was written by Dmitri Shostakovich in 1951 (opus 86), with words by Yevgeniy Dolmatovsky."
Admittedly whistling isn't singing, but as he was the first, by several years, and did it all alone, he gets my vote ?
"99 million bottles of beer on the wall, 99 million bottles of beer, if one of those bottles should happen to fall..."
Hey, it was a long trip and they had to do something to pass the time.
This is a guess - It’s a wonderful World by Louie Armstrong! Cheers!