Hamlet (1.2)

"My father's spirit-in arms! All is not well. / I doubt some foul play. Would the night were come! / Till then, sit still, my soul. [Foul] deeds will rise, / Though all the earth o'erwhelm them, to men's / eyes" (Shakespeare 1.3.277-281)

Horatio, Marcellus, and Barnardo have told Hamlet that they have seen his father's ghost, and they believe that the ghost wants to se his son. Hamlet decides that he will join them in their shift that night to see his father's ghost with them. When they leave, Hamlet notes that something must be wrong if his father was wearing his arms. In addition, Hamlet suspects some treacherous action might have taken place, whether it pertains to the impending battle against young Fortinbras, or whether it pertains to King Hamlet's death. Hamlet is visibly excited to see his father again, which shows that he must have been close to him; in addition, Hamlet's reaction to what the ghost was wearing is an indication that he knew his father very well. It'll certainly be interesting to see whether the ghost will talk to Hamlet or not, and what the ghost might say if it does.