最初の行の正規表現

$ cat file
pool 0.ubuntu.pool.ntp.org iburst maxsources 1

$ sed -e 's/^\(.*pool\.ntp\.org\)/#\1/' file
#pool 0.ubuntu.pool.ntp.org iburst maxsources 1

説明：

 ^                   # beginning of line
  \(                  # start group 1
    .*                  # 0 or more any character
    pool\.ntp\.org      # literally
  \)                  # end group

置換：

 #           # character #
\1          # backreference to group 1