John 5:18

18Because ofδιὰthisτοῦτοthereforeοὖνthe moreμᾶλλονwere seekingἐζήτουνHimαὐτὸνtheοἱJewsἸουδαῖοιto killἀποκτεῖναιbecauseὅτιnotοὐonlyμόνονwas He breakingἔλυεtheτὸSabbathσάββατον,butἀλλὰalsoκαὶFatherπατέραHis ownἴδιονHe was callingἔλεγετὸνGodθεόν,equalἴσονHimselfἑαυτὸνmakingποιῶνtoτῷGodθεῷ.

Therefore because of this, the Jews were seeking more to kill Him, because not only was He breaking the Sabbath, but He was also calling God His own Father, making Himself equal to God.