When did Jesus say not to let your left hand know what your right hand is doing?

Answer: When giving to the needy

Jesus taught giving in secret: when you give to the needy, do not let your left hand know what your right hand is doing.

“But when you give to the needy, do not let your left hand know what your right hand is doing, so that your giving may be in secret. And your Father, who sees what is done in secret, will reward you.”— Matthew 6:3-4 (BSB)