Ahmed, who is an electrician, perform services for a client December 1, 2019. Ahmed billed the client $1,500 on December 5, 2019. Immediately after that, Ahmed traveled out of town and won't return until January 7, 2020. Ahmed authorized his friend Mary to act as his agent to receive the payment. Ahmeds client is aware of this arrangement. Mary received the payment December 30, 2019. Ahmed deposit the $1,500 on January 8, 2020.

When should Ahmed recognize the $1,500 for tax purpose? Explain?