Drop source value - Problem with filter

I am new to ES and logstash. I am getting some unwanted logs with source : "horizon-error" in Kibana. I want to drop those messages. Have tried below option, but no luck. Would really appreciate any help.

if "horizon" in [source] { drop{} } }

This is how my logstash.conf looks like.

input {
syslog { port => 5544 codec => json type => "infra" } }
filter {
if "horizon" in [source] { drop{} } }
output {
elasticsearch { hosts => ["localhost:9200"] index => "infra-%{cloudname}-%{+YYYY.MM.dd}" } }

Never mind. I figured out after spending quite some time, a silly mistake. My field was "@source" , but I was trying to put condition on "source".

if "horizon" in [@source] { drop{} } } ----- Worked.