Let me have men about me that are fat,I just came across the source of that passage. It is from Plutarch's Parallel Lives, specifically, The Life of Antony.
Sleek-headed men and such as sleep a-nights.
Yond Cassius has a lean and hungry look.
He thinks too much. Such men are dangerous.
When Caesar returned from Spain, all the principal men went many days' journey to meet him, but it was Antony who was conspicuously honoured by him. For as he journeyed through Italy he had Antony in the same car with himself, but behind him Brutus Albinus, and Octavius, his niece's son, who was afterwards named Caesar and ruled Rome for a very long time. Moreover, when Caesar had for the fifth time been appointed consul, he immediately chose Antony as his colleague. It was his purpose also to resign his own office and make it over to Dolabella; and he proposed this to the senate. But since Antony vehemently opposed the plan, heaped much abuse upon Dolabella, and received as much in return, for the time being Caesar desisted, being ashamed of their unseemly conduct. And afterwards, when Caesar came before the people to proclaim Dolabella, Antony shouted that the omens were opposed. Caesar therefore yielded, and gave up Dolabella, who was much annoyed. And it would seem that Caesar abominated Dolabella also no less than he did Antony. For we are told that when a certain man was accusing both of them to him, he said he had no fear of those fat and long-haired fellows, but rather of those pale and thin ones, indicating Brutus and Cassius, by whom he was to be conspired against and slain.