There has long 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.
On Tuesday, Casillas called time on his career, which started with a Madrid debut in 1999 and saw him rack up 725 appearances, five LaLiga titles and three Champions League medals before departing for Porto in 2015.
Buffon is still going strong at the age of 42, albeit he departed Juve for a season at Paris Saint-Germain before returning last year and now largely acts as understudy to Wojciech Szczesny.
While Buffon has never experienced Champions League success with Juve, both he and Casillas have won the World Cup with Italy and Spain respectively.
Buffon credits Casillas for making him a better goalkeeper.
"They say competition makes us better than others but not perfect faced with ourselves," Buffon wrote on Twitter.
"Maybe this futile pursuit of perfection is what made us who we are.
"#Grac1as Iker, without you, everything would have been less meaningful."