مِنْ حُسْنِ إِسْلَامِ الْمَرْءِ تَرْكُهُ مَا لَا يَعْنِيهِ (Tirmidhi, Ahmad, and others)
Translation: Part of the excellence of a person's Islam is to leave what does not concern him.
حُسْنِ — excellence or goodness; الْمَرْءِ — a person; تَرْكُهُ — his leaving; مَا لَا يَعْنِيهِ — what does not concern him.
Brief grammar: In تَرْكُهُ, تَرْك is a verbal noun, and هُ means “his.” مَا means “that which/what,” and لَا يَعْنِيهِ describes it.
The advice concerns needless intrusion, idle curiosity, and pointless speech. Helping someone when responsibility or compassion calls for it is not excluded by this teaching.