Wick theorem for $N$-particle Matsubara green function: equal time contraction

Question

I am wondering why, e.g., the book by Mahan, "Many-particle physics", mentions that contractions in Wick theorem for the $N$-particle Matsubara Green function between a pair of operators at equal times must be computed by putting the creation operator to the left of the annihilation one. Where does this rule come from? Why is it correct, if time-ordering is not well defined for equal times?