"If, in the blind and fatal mistake of sectional antagonism or partisan bitterness, this most infamous of human crimes found apologists, there were, at least, some notable exceptions to this feeling. Thus, when the tidings reached Richmond, General Lee at first refused to hear the details of the horrid deed, from the two gentlemen who waited unon him on Sunday night with the particulars. He said, that when he dispossessed himself of the command of the confederate forces, he kept in view President Lincoln's benignity, and surrendered as much to the latter's goodness as to Grant's artillery. The general said that he regretted Mr. Lincoln's death as much as any man in the north, and believed him to be the epitome of magnanimity and good faith." 1879

"We know full well that many Southerners sincerely disown the fiendish crime—that Ould spake truth when he said that it was the greatest blow the South had sustained ; and the chivalric Lee no less so, when he declared that he surrendered as much to the goodness of Lincoln as to the prowess of Grant, and that the deceased President was the most magnanimous and kindhearted man that ever breathed." 1865

I cannot find a direct quote from Lee.