Casillas is leaving Spanish giants Real Madrid for Portuguese outfit FC Porto, having won multiple honours with Los Blancos.
The 34-year-old Spain international, who is leaving Madrid after 25 years at the club, is widely considered as one of the greatest goalkeepers of all time.
Bale, who joined Madrid from Tottenham in the summer of 2013, hopes that Casillas succeeds at Porto and is honoured to have played with him for two years.
Been an absolute honour playing alongside @CasillasWorld the past 2 years. A true legend of the game and I wish you all the luck my friend!— Gareth Bale (@GarethBale11) July 12, 2015
Will Gareth Bale be next to leave Real Madrid?
Bale had a successful 2013-14 campaign with Madrid in which he won the Copa del Rey and the Champions League, scoring in the finals of both the competitions.
The former Southampton star struggled with injuries and poor form for much of last season, though, scoring 13 goals and creating 49 chances in 31 La Liga appearances.
Manchester United are reportedly interested in signing the Wales international this summer, with a recent report in Spanish news outlet Cope claiming that the Premier League giants had a bid of €140 million (£100.9m) for him rejected.
However, it is unlikely that Bale will leave Madrid this summer. The 25-year-old winger recently tweeted his aim to win the Champions League with the Spanish powerhouse next season.