The holy name

Never speak ill of devotees

Insulting the devotees who have given their lives to spreading the holy name is the first of the ten offenses to avoid while chanting.

The lists of offenses to the holy name begin with insulting devotees, in particular those who have given their lives to spreading the name of the Lord. Belittling such a devotee spoils the very chanting you are doing. When you disagree with someone, speak to them directly and kindly, or leave it. Do not talk about them behind their back.

Whose teaching
Padma Purāṇa
Where it is taught
The ten offenses to the holy name
Source
ISKCON Bangalore
Never speak ill of devotees · HariHariBol