Try:
Bidding.where('listing_id = :listing_id', listing_id: 1).maximum(:bid_amount)
Update:
To follow up on your comment: since you say you are passing in params[:id]
, it's best to convert that parameter to integer so that unwanted values don't go to the database. For e.g.
Bidding.where('listing_id = :listing_id', listing_id: params[:id].to_i).maximum(:bid_amount)