Conduct tax research to address the following fact pattern:

A cash basis individual taxpayer has a freelance business by which he provides bookkeeping services.  On December 31, 2020, the client had a balance due to the taxpayer/freelancer for $500 for work performed in December 2020.  The client called the taxpayer on December 31, 2010 to let him know that the check for $500 was available for him to pick up to pay off his balance.  However, the taxpayer was busy with personal projects during the day.  The drive to pick up the check and return back home would have taken about 2-3 hours all together since he lives 60 miles away from the client and the traffic to the client's was usually heavy during the week.  Notwithstanding these facts, taxpayer had no severe restrictions that would have prevented him from picking up the check.  The taxpayer asked the client to mail the check to his home.  The client mailed the check to taxpayer's home on December 31st.  It was received by the taxpayer on January 5, 2021.

The taxpayer is putting together his tax records to prepare his taxes.  As he is preparing his taxes, he asked you, as the tax professional, on whether he should include the $500 as part of his 2020 tax year income taxes rather than report it on next year's taxes for 2021 since he received the check in calendar in 2021.

What would be your response to his inquiry?  Please provide legal authority either from the Internal Revenue Code or case law.  And describe how you went about to find support for your response.  Don't rely on Googling the answer.   The point of this exercise is to learn how to conduct proper legal research.