Spain's World Cup-winning goalkeeper Iker Casillas says "everything is controlled" after he had a heart attack during training with his club Porto.The 37-year-old is "stable" in hospital and remains under observation.Casillas, regarded as one of Real Madrid's greatest keepers, says he had "a big scare" but added that his "strength was intact".A Porto statement said he "suffered an acute myocardial infarction" on Wednesday morning."The work session was promptly interrupted to provide assistance to the Porto goalkeeper, who is currently at the Hospital CUF Porto. Casillas is well, stable and his heart problem [is] solved," it added.On Wednesday evening, Casillas tweeted "everything has been controlled" and thanked "everyone for the messages and affection", alongside a picture from his hospital bedCasillas made 725 appearances for Spanish giants Real during a 16-year career at the Bernabeu, helping them win three Champions Leagues and five La Liga titles.He was also part of the Spain squad as they won two successive European Championships, in 2008 and 2012, and the World Cup in 2010.Madrid said they were sending their former keeper "all the courage of the world"."Real Madrid wants to transmit all their support to their beloved captain Casillas," the statement read."Casillas has taught us throughout his professional career to overcome the most incredible challenges to enhance the glory of our club."It has taught us that giving up does not fit into our philosophy of life and has shown us countless times that being stronger, the harder the challenge, is the way to achieve victory."Real Madrid want to see their eternal captain recovered as soon as possible and send him all the courage of the world."