Or:

=SUM(INDIRECT("Sheet1!$C" & (ROW(L26)-26)*7+1 &" :$BB"& (ROW(L26)-26)*7+7))

Or:

=SUM(INDIRECT("Sheet1!$C" & (ROW(L26)-ROW(L$26))*7+1 &" :$BB"& (ROW(L26)-ROW(L$26))*7+7))

Depending on whether or not you want to see the numbers change.

Regards