Consider the TRS R consisting of the rewrite rule

1: dfib(s(s(x)),y) -> dfib(s(x),dfib(x,y))

There are 2 dependency pairs:

2: DFIB(s(s(x)),y) -> DFIB(s(x),dfib(x,y))
3: DFIB(s(s(x)),y) -> DFIB(x,y)

Consider the SCC {2,3}.
By taking the polynomial interpretation
[s](x) = x + 1,
[DFIB](x,y) = x + y + 1
and [dfib](x,y) = y,
rule 1
is weakly decreasing and
the rules in {2,3}
are strictly decreasing.
Hence the TRS is terminating.