Babylonian Talmud, Ketubot 50a

R. Elai stated: It was ordained at Usha that if one wishes to spend liberally one should not spend more than a fifth. So it was also taught: If one desires to spend liberally one should not spend more than a fifth, [since by spending more] one might oneself come to be in need [of the help] of people. It once happened that a man wished to spend more than a fifth but his friend did not allow him. Who was it? — R. Yeshebab. Others say [that the man who wished to spend was] R. Yeshebab, but his friend did not allow him. And who was it? R. Akiba. R. Nahman, or as some say, R. Aha b. Jacob, said: What [is the proof from] Scripture? — And of all that You give me I will set side a tithe for You. (Genesis 28:22) [Translation by Josh Yuter. Edited for gender neutrality]