Parashas Naso · Passuk 5:10
And each shall retain his sacred donations: each priest shall keep what is given to him.
Chabad.org — English translation + Rashi
Source: Bava Kamma 110b.
English: “It shall be his” — R’ Yochanan said: Anyone who has terumos and maasros and does not give them to the kohen will ultimately need them [himself].
In context: Withholding the priestly gifts is midrashically linked to future need; the gifts that should have been given will be required by the owner in distress.
Custom
Chazal warn that one who withholds terumah and maaser will eventually find himself in need of those very gifts.