Former President George H.W. Bush and his wife, Barbara, at Game 3 of the American League Division Series in 2015. AP Photo/David J. Phillip

Former President George H.W. Bush wrote a note to President-elect Donald Trump explaining why he won't attend the inauguration on Friday, saying that his doctor told him that sitting outside in January "likely will put me six feet under."

The letter was posted by Bush's spokesman, Jim McGrath, on Twitter on Wednesday morning. It's dated January 10.

"Barbara and I are so sorry we can't be there for your inauguration on January 20," Bush wrote. "My doctor says if I sit outside in January, it likely will put me six feet under. Same for Barbara. So I guess we're stuck in Texas."

The former president wished Trump "the very best" and said he'd be with Trump and the country "in spirit."

Bush was hospitalized in Houston on Saturday for respiratory problems stemming from pneumonia, according to McGrath. He's in stable condition after doctors cleared his airway, and he'll remain under observation in the intensive care unit. His wife, Barbara, was also admitted to the hospital on Wednesday morning after experiencing fatigue and coughing.

The former president is 92 years old.