hundreds of kehillos kedoshos not saying tachanun by mincha every day would constitute a source for this minhag
The minhag of many Chassidish kehilos is not to say tachanun at mincha. This minhag is recorded in many Chassidic and Sefardic sources. A number of explanations are offered:
1 they often daven until after shkiya, and according to many opinions tachanun may not be said after shkiya, so a blanket rule was instituted so as never to come to saying after shkiya, which in some kabbalistic sources danger is associated with this practice.
2 tachanun by mincha requires intense concentration, which most people don’t have in the middle of their day
3 after it is night in Israel tachanun should not be said even in other parts of the world
Sources:
Sefer Nimukei Orach Chaim, Shu”T Dvar Yehoshua Y:D 3:74, Zivchei Zedek siman 9 [minhag bagdad], Sefer Shulchan Hatohar Siman 22.