I'm new with the U test and I have some doubts about the rejection of the null hypothesis with the U test with normal distribution approximation.
The Mann-Whitney U test is not of itself a test of the hypothesis of equality of population medians -- at least no more than it is a test of equality of population means. If accompanied by additional assumptions (that would make what it does test equivalent to a test of equality of populations) then it can function as such a test.
I obtained Z=1,0313
Do I reject the null hypothesis if
Possibly neither of those -- it depends on exactly what the statistic you have is computing. Note, however, that it would be very unusual indeed to have a test where the rejection region included a Z-score of 0 and this is not one of those rare cases.
If your $U$ is the number of times an observation from sample 1 exceeds an observation from sample 2 (what I'd expect to be the most likely definition of $U$) then you'd reject for unusually small values of the statistic, so most likely you'd actually reject for $Z\leq -Z_α$.
However, you should double check how $U$ has actually been defined for your situation (and strictly speaking, you should also double check how $Z$ is defined in terms of that $U$ as well). That would then confirm which direction the comparison should go (i.e. whether the rejection region is $Z\leq -Z_α$ or $Z\geq Z_α$).