It comes from article 132, section 1 of the Withdrawal Agreement (WA), which states that:
Notwithstanding Article 126, the Joint Committee may, before 1 July 2020, adopt a single decision extending the transition period for up to 1 or 2 years.
(Article 126 states that the implementation period ends on 31 Dec 2020.)
However, the new European Union (Withdrawal Agreement) Bill 2019-20 (WAB) states, in clause 33, that:
A Minister of the Crown may not agree in the Joint Committee to an
extension of the implementation period.
(Note that this clause was not present in the version of the WAB which started (but failed to complete) its passage through Parliament before the 2019 election.)
This can't be sidestepped by the government sending an official or other representative instead of a minister, because clause 34 specifies that only a Minister of the Crown can be the UK's co-chair of the committee.
Because "The Joint Committee shall adopt its decisions and make its recommendations by mutual consent" (article 166, section 3), this seems to imply that if a co-chair objects to a proposal, that is as good as a veto.
So in other words: what the WA gives, the WAB takes away.