Cassius Dio, Roman Histories  44.16  (ca. A.D. 220):

16.  It had been decided by them to make the attempt in the senate, for they thought that there Caesar would least expect to be harmed in any way and would thus fall an easier victim, while they would find a safe opportunity by having swords instead of documents brought into the chamber in boxes, and the rest, being unarmed, would not be able to offer any resistance.  But in case any one should be so rash, they hoped at least that the gladiators, many of whom they had previously stationed in Pompeius' Theater under the pretext that they were to contend there, would come to their aid; for these were to lie in wait somewhere there in a certain room of the peristyle.  So the conspirators, when the appointed day was come, gathered in the senate-house at dawn and called for Caesar.  (E. Cary, trans.)