That emerged when Da Costa was discussing the goalkeeping great's bid to become president of the Royal Spanish Football Federation - a process from which Casillas later withdrew.
"I think I can say, without hesitation, that it has been the path and the dream destination".
During his time with the national team, he led them to win the 2010 World Cup and two successive European Championships in 2008 and 2012.
He was a regular for the Portuguese club until he suffered a heart attack in training on May 1, 2019.
In a glittering career, Casillas won three Champions Leagues and five LaLiga titles with Madrid, while he was a two-time European Championship victor with Spain, who he also captained to World Cup glory in 2010.
Casillas played four seasons at Porto and has was first choice before being forced to the sidelines.
"Real Madrid CF wants to give recognition and show its admiration and feelings for one of the biggest legends in our club and in world football", the club's statement says.
Near the end of his run with Real Madrid, Casillas fell out with Mourinho, who dropped his side's long-time starter in favour of Diego Lopez.
As one of the world's greatest-ever shot-stoppers, Casillas has finally called time on a trophy-laden career spanning more than 20 years, during which he won 27 trophies.
Casillas, who spent most of his career with Real Madrid made 725 appearances for the Spanish giants.
"In Portugal, with Porto, he has achieved two Leagues, one Cup and one Super Cup".
Casillas has been awarded the La Liga Best Goalkeeper gong twice, been voted into the FIFA FIFPro World XI five times, won the World Cup Golden Glove in 2010 and was named Primeira Liga Best Goalkeeper for 2019.
There has always been a mutual respect between veteran Juventus legend Buffon and Real Madrid great Casillas, who have earned reputations as two of the greatest goalkeepers of their generation. "Real Madrid would like to convey to him and his family the love of our club, which is the club of his life", Real Madrid's statement concluded.