Is it forbidden to rely on a well-off father, stop working, and practice sports?

Your father is not obligated to spend on you if you have reached adulthood and are capable of earning. However, if he spends on you voluntarily, there is no harm in it. You are advised to work and earn to lighten the burden on your father, as idleness and free time can lead to sins. Ibn Mas'ud said: "I indeed detest the man I see idle, neither engaged in matters of this world nor in matters of the Hereafter."