During Tuesday night's debate on whether US President Donald Trump will ever disclose his tax returns, Democratic presidential candidate Joe Biden cast doubt.

"Trump said repeatedly as moderator Chris Wallace pushed him to commit to a definite timetable," You'll get to see it. Biden replied, 'When? From Inshallah? While it translates to "God willing," the Arabic language phrase also has colloquial connotations of ambiguous commitment.

Biden previously released his personal income taxes, which show that about 30 percent of his $985,000 gross personal income was paid by the former vice president and his wife, Jill Biden.

On the other hand, Trump has declined to report his income tax returns freely, a presidential tradition going for decades.

The New York Times reported on Monday that in 10 of the past 15 years, Trump has not paid any federal income taxes.

It said that in 2016, the former businessman paid only $750 and another $750 in 2017, the year he took office.

Trump disputed the report during Tuesday night’s debate, saying he has “paid millions of dollars in taxes, millions of dollars of income tax”.

Social media users were quick to react with humour at Biden’s “inshallah” remark, with many Twitter users posting their reactions instantly.

Some referred to it as an “historic moment in America”.

Others wondered whether Biden had said “in July” rather than the common Arabic expression.

However, Asma Khalid, a national political correspondent for NPR covering the 2020 elections, said she confirmed it with Biden’s campaign. And some expressed their surprise while noting that “anything is possible in 2020”, a year marked with unprecedented events.