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Every $SU(2)$ transformation can be written as $$ g(x) = a_0(x) 1 + i a_i(x) \sigma ,$$ where $\sigma$ are the Pauli matrices. The defining conditions of $SU(2)$ are $g(x)^\dagger g(x)=1$ and $det(g(x)=1$, and thus we have $$ (a_0)^2 +a_i^2=1 , $$ which is the defining condition of $S^3$.
Source: page 23 in http://www.iop.vast.ac.vn/theor/conferences/vsop/18/files/QFT-4.pdf
This is also shown nicely at page 164 in the book Magnetic Monopoles by Shnir.