##Actually, 1 byte

f

Yes, there's a builtin for this, since November 16, 2015.

Try it online

For fun, without the builtin, it's 9 bytes:

╗1`F╜=`╓i

Try it online!

Explanation:

╗1`F╜=`╓i
╗          push input to register 0
 1`F╜=`╓   push list containing first value x (starting with x = 0) where:
   F         fib(x)
    ╜=       is equal to the input
        i  flatten the list

Actually, 1 byte

f

Yes, there's a builtin for this, since November 16, 2015.

Try it online

For fun, without the builtin, it's 9 bytes:

╗1`F╜=`╓i

Try it online!

Explanation:

╗1`F╜=`╓i
╗          push input to register 0
 1`F╜=`╓   push list containing first value x (starting with x = 0) where:
   F         fib(x)
    ╜=       is equal to the input
        i  flatten the list

##Actually, 1 byte

f

Yes, there's a builtin for this, since November 16, 2015.

Try it online

##Actually, 1 byte

f

Yes, there's a builtin for this, since November 16, 2015.

Try it online

For fun, without the builtin, it's 9 bytes:

╗1`F╜=`╓i

Try it online!

Explanation:

╗1`F╜=`╓i
╗          push input to register 0
 1`F╜=`╓   push list containing first value x (starting with x = 0) where:
   F         fib(x)
    ╜=       is equal to the input
        i  flatten the list