Allaah (subhaanahu wa ta'aalaa) states in the Glorious Qur'aan:
وَيَوْمَ يَعَضُّ الظَّالِمُ عَلَى يَدَيْهِ يَقُولُ يَا لَيْتَنِي اتَّخَذْتُ مَعَ الرَّسُولِ سَبِيلاً يَا وَيْلَتَى لَيْتَنِي لَمْ أَتَّخِذْ فُلَاناً خَلِيلاً لَقَدْ أَضَلَّنِي عَنِ الذِّكْرِ بَعْدَ إِذْ جَاءنِي وَكَانَ الشَّيْطَانُ لِلْإِنسَانِ خَذُولاً
"And (remember) the Day when the wrong-doer will bite on his hands, saying: "Oh! Would that I had taken a path with the Messenger. Ah! Woe to me! Would that I had never taken so-and-so as a friend! He indeed led me astray from the Reminder [the Qur'aan] after it had come to me. And satan is ever a deserter to man in the hour of need."(1)
Keeping righteous practicing Muslim friends is essential for maintaining a healthy spiritual environment in our daily life. Friends affect our Eemaan level. If they are righteous, they will inspire us to increase our knowledge and Eemaan. Conversely, an effective way to downgrade our Eemaan is keeping friends who smoke, drink alcohol, or never go near religion (maybe except when making a religious 'joke').
"It doesn't matter if my friends drink alcohol. That is not going to affect me," some might say. But first, these Muslims are sharing in their friends' evil by simply "tolerating" their drinking habit. And second, their unique power of staying clean of alcohol while their friends drink can only hold weight in their heads. In weeks, or months, they will get used to the behavior of their friends. They will start considering it normal, maybe even cool. And when that happens, during a moment of depression or excitement, they will be pushed into drinking by the encouragement of their great friends.
The Prophet (salla Allaahu 'alayhy wa sallam) best described the similitude of the good and bad companions when he said:
مَثَلُ الْجَلِيسِ الصَّالِحِ وَالْجَلِيسِ السَّوْءِ كَمَثَلِ صَاحِبِ الْمِسْكِ وَكِيرِ الْحَدَّادِ لَا يَعْدَمُكَ مِنْ صَاحِبِ الْمِسْكِ إِمَّا تَشْتَرِيهِ أَوْ تَجِدُ رِيحَهُ وَكِيرُ الْحَدَّادِ يُحْرِقُ بَدَنَكَ أَوْ ثَوْبَكَ أَوْ تَجِدُ مِنْهُ رِيحًا خَبِيثَةً"The example of a good companion and a bad companion is that of the bearer of Musk and the worker on the bellows. The bearer of Musk would give you some of the perfume, you either buy it or you smell its fragrance. The worker on the bellows, on the other hand, either burns your body or your clothes, or you smell a bad odor from him."(2)
Look at your friends; are they bearers of Musk or workers on the bellows?
(1) Surah al-Furqaan, 27-29
(2) Reported by al-Bukhaaree in his Saheeh (1959 and 5108) and Muslim in his Saheeh (4762)
Other posts by Rawa: Knowing Allaah at All Times - First thing you will be asked about on the Day of Judgment