In the vote on the Withdrawal Agreement Bill held eight days after the 2019 election, on the 20th of December 2019, newly elected Labour MP Charlotte Nichols broke a three-line whip and abstained on the bill. Using the swearing-in arithmetic in CDJB's answer, this was just 1 day, 22 hours and 28 minutes after being sworn in on the 18th of December.
On Twitter, she gave this explanation of her actions -
I never thought my first vote as a Labour MP would be breaking a three-line whip, but I owe it to my constituents and to members of my Constituency Labour Party to explain my actions, and I hope you can understand it was a decision I agonised over.
The country voted to leave the European Union, and Warrington North voted to leave ahead of the national picture. I respect the result of the Referendum, and the strength of feeling locally was very clear at every door I knocked (even among many of those who had voted Remain in 2016) that the last few years of indecision have been bad for our country and we needed to get on with leaving, and bringing the country back together. Many felt like Labour were frustrating the process of leaving not because we wanted the best possible deal for our country but because we didn't want to leave at all. It was this lack of trust that nationally, I believe, cost us the election.
The Withdrawal Agreement put before us today, however, had many fundamental differences with the Withdrawal Agreement tabled just before the General Election. While I believe this Government does have a clear mandate to deliver Brexit, I do not believe they have any mandate or any justification for these changes- including to employment protections and transitional trading
I did not feel I could follow the whip and vote against the Bill, having listened to my constituents wishes. I do not wish to frustrate the process, so I did not stand in its way. I could not, however, in good faith vote for the Withdrawal Agreement as amended since the election. So I have abstained in the hope the Government, in the spirit of co-operation and bringing the country together, will listen to members from across the House who have raised concerns about these new amendments and will work collegiately in the committee stages before the Bill comes back for its next reading. I hope it was the right decision.
Fellow new Labour MP, Abena Oppong-Asare, also abstained on the bill, but this was apparently due to her mistakenly missing the vote. Zarah Sultana, another new Labour MP, also abstained - but I can't find any explanation for her absence, so it's possible this was a paired vote or an authorized absence. In any case, Nichols was sworn in after both of them, so keeps the record.