Abdullah ibn Mas‘ud (radiyallahu ‘anhu) said:

اعتبروا الناس بأخدانهم، فإن المرء لا يخادن إلا من يعجبه

“Judge people by their friends, for a person only befriends those whom they like.”

Shaykh Salih Aal al-Shaykh (hafidhahullah):

This is taken from the authentic hadith that the Prophet ﷺ said:

الرَّجُلُ عَلَى دِينِ خَلِيلِهِ فَلْيَنْظُرْ أَحَدُكُمْ مَنْ يُخَالِلُ

“A man is upon the religion of his close friend, so let each of you look to see who he is taking as his friend.” (Abu Dawud & al-Tirmidhi)

So, Ibn Mas‘ud (radiyallahu ‘anhu) says: “Judge people by their friends, for a person only befriends those whom they like.” — They like their behavior, they like their intellect, they like their way of thinking. So, if you see someone befriending another person, meaning being a close friend to them, consider this person as akin to that one. The Prophet ﷺ said:

الأَرْوَاحُ جُنُودٌ مُجَنَّدَةٌ, فَمَا تَعَارَفَ مِنْهَا ائْتَلَفَ, وَمَا تَنَاكَرَ مِنْهَا اخْتَلَفَ

“The souls are troops gathered together. They come close upon what they recognise, and they differ upon what they reject.” (al-Bukhari)

If you find someone who enjoys the company of sinners, even if they appear outwardly righteous, it is likely that their inner self is inclined towards sin, even if subtly.

If you find someone who keeps the company of scholars, you will find that their inner self is inclined towards knowledge, even if they are not students of knowledge.

If you find yourself keeping the company of the people of Sunnah, it means that your heart loves it.

If you find yourself keeping the company of innovators, gossipers, tale-carriers and slanderers then know that a person is on the religion of their close friend. Thus, you should begin rectifying yourself.

This statement of Ibn Mas‘ud (radiyallahu ‘anhu) is for you to judge yourself and others. It is a cultivating and comprehensive piece of advice, and everyone must hold themselves accountable. Allah has informed us that people will say about their friends on the Day of Judgement:

“Oh, woe to me! I wish I had not taken so-and-so as a friend. He led me away from the remembrance after it had come to me. And ever is Shaytan, to man, a deserter.” (al-Furqan 25:28-29)

Source: Lecture entitled “Words of Advice from Ibn Mas‘ud”, at minute 34.