Permissiblity up to 5% of interest income proof

Assalamu Alaikum

Happened to discuss stock market trading with one of the great scholar of my city(India) He is of the opinion stock trading is haram. The main contention is even very small amount of interest income is not permissible, where as those scholars who permit stock trading they say income from interest upto 3% of total income is permissible (reference Can you please enlighten based on which hadees or ijtihad it is permissible up to 3%
Gave him below reference

Yet he wants to know what is the basis of the ijtihad