An article published on Nature journal is the holy grail of biomedical research. Like this one, titled "Estimating the effects of non-pharmaceutical interventions on COVID-19 in Europe"
https://www.nature.com/articles/s41586-020-2405-7
It fits the bill bill and states "Lockdown has an identifiable large effect on transmission (81% (75–87%) reduction)"
Not a popular statement to make, but it is what it is.