"If you do not do 996 when you are young, when will you? Do you think never having to work 996 in your life is an honour to boast about? If you join Alibaba, you should get ready to work 12 hours a day, otherwise why do you come to Alibaba? We do not need those who comfortably work eight hours."
He added, "If you do not pay the price, how will you gain?"
Ma said that without the 996 system China's economy was "very likely to lose vitality and impetus". JD.com founder Richard Liu backed his stance, saying China's rapid growth had increased the number of "slackers".
JD.com, founded in 1998, today has a market cap of nearly $US43 billion. Alibaba, founded the following year, is worth $US475 billion and is one of the world's biggest internet companies.
Liu shared his thoughts about his attitude to work in an online post amid reports the company was cutting jobs. He said he used to set his alarm to wake him up every two hours so he could offer customers 24-hour service.
"JD in the last four, five years has not made any eliminations, so the number of staff has expanded rapidly, the number of people giving orders has grown and grown, while the those who are working have fallen," Liu wrote, according to the BBC.
"Instead, the number of slackers has rapidly grown! If this carries on, JD will have no hope! And the company will only be heartlessly kicked out of the market! Slackers are not my brothers!"
Ma on Sunday appeared to backtrack, describing long hours as "unsustainable".
"If you find a job you like, the 996 problem does not exist — if you're not passionate about it, every minute of going to work is a torment," he said in a post on Weibo, the South China Morning Post reported.
"No one likes working at a company that forces you to do 996. Not only is it inhumane, it's unhealthy and even more unsustainable for long periods — plus workers, relatives and the law do not approve of it. In the long term, even if you pay a higher salary, employees will all leave."
Ma said companies that forced staff to work overtime were "foolish" and doomed to fail, adding the "real 996 should be spending time learning, thinking and for self-improvement".
"The people who stick to 996 must have found their passion there, and their happiness besides from money,' he said.
China's official labour law prohibits more than 36 hours of overtime a month.
The Github group last week had attracted more than 200,000 "stars" indicating support, prompting some Chinese web browsers in the notoriously censorship-heavy country to block access to the post.
But the ruling Communist Party's official newspaper the People's Daily on Saturday weighed into the debate, saying people who raised concerns about 996 should not be "labelled", according to the South China Morning Post.
"Valuing hard work does not equate to forcing employees to work overtime," the editorial said. "One should not attach the moral labels of 'slackers' or 'not willing to strive' to employees who are against 996."
