Casillas is in a stable condition in hospital and had an operation to resolve the cardiac problem he had.
The former Real Madrid goalkeeper uploaded a photo of himself in his hospital bed to Instagram and Twitter.
"Everything is under control here," read the caption. "A big scare but still going strong. Thank you very much for all your messages and love."
This comes after the message of "I'm fine," which he sent to his loved ones earlier in the day.