General Longstreet was delayed by a force occupying the high, rocky hill on the enemy’s extreme left, from which his troops could be attacked from reverse as they advanced. His operations had been embarrassed the day previously from the same cause, and he now deemed it necessary to defend his flank and rear with the divisions of Hood and McLaws. He was therefore reinforced by Heth’s division and two brigades of Pender’s, to the command of which Major-General Trimble was assigned. General Hill was directed to hold his line with the rest of the command, to afford General Longstreet further assistance if required, and to avail himself of any success that might be gained.
Source: From Manassas to Appomattox
This piece of the record is important due to statements by members of Lee's staff, most notably Walter Taylor, who claim Longstreet's flank was safely situated and that they had heard Longstreet ordered to include McLaws and Hood in the general assault. This part of the report, however, indicates that Lee was aware that Hood and McLaws would not be taking part in the general assault, thus the need for reinforcements from Heth's and Pender's divisions.
Most sources seem to indicate -- and the above statements from Lee support -- the notion that Lee's original order was for the entire First Corps to make the assualt on Cemetery Ridge, but after hearing Longstreet's objections of the troops to be used, Lee allowed Longstreet to keep Hood and McLaws out on his flank to guard against attack from that quarter. To make up for the loss of troops, Lee took men from Heth's and Pender's divisions placing them under the command of Trimble and Pettigrew.
Commentary
Portion of General Lee’s report of the Battle of Gettysburg for July 3, 1863 relating to the use of Hood’s and McLaws’ divisions to guard Longstreet’s right flank for the advance to Cemetery Ridge.