It states (Shemos 19:2) ההר נגד ישראל שם ויחן, "Yisrael encamped there opposite the mountain. Rashi writes, "[The singular form of ויחן, instead of the plural ויחנו, denotes that they encamped] as one man with one heart, but all the other encampments were with complaints and with strife." So, when the Jewish nation came to Har Sinai, they were united. The Or HaChaim explains that unity is a prerequisite for succeeding in Torah study.
Unity is also a great help with our battle against the yetzer hara.
There was a chasid who was tempted to commit a severe aveirah, but stopped himself when he realized that after committing the aveirah, he wouldn't be able to face his Rebbe anymore. But then he told himself he would commit the aveirah and stop going to his Rebbe. Later, he said to himself, "If I don't go to the Rebbe, I will miss seeing all my friends who go to the Rebbe. How can I miss meeting with all my friends?" That thought stopped him from the aveirah.
The next time he came to his Rebbe, the Rebbe said (with ruach hakodesh), "What a Rebbe couldn't achieve, the chassidim achieved." (Because the thought of not seeing the Rebbe didn't stop him from aveiros, but the thought of losing his friends saved him from the aveirah) . This is an example of the good influence we receive from unity and having good friends. It is our strength against the yetzer hara.
Reb Akiva Eiger zt'l says that the הר, mountain, represents the yetzer hara (see Succah 52.) . So, ההר נגד ישראל שם ויחן means that when the Jewish nation is united, they can conquer the yetzer hara.
The Midrash (Yalkut Shimoni 940) states, "אתם נצבים , you are standing [and no one can harm you]. When? כולכם, when you are united... A child can break a single reed, but no one can break many reeds bundled together." When you are united, you are a force that the yetzer hara can't break.
The Beis Aharon zt'l once sat with his chasidim at a tish, and he told one of his followers to go outdoors and return to report what he saw.
The chassid returned and said, "I saw two drunkards walking down the street. One said, "Let's hold on to each other so we don't fall."
The Beis Aharon zt’l said, "Did you hear what he said? If we hold on to one another, we won't fall!”
Unity helps us serve Hashem with hislahavus. When one piece of wood catches on fire, the fire will extinguish quickly. But if there are several pieces of wood and one of them catches fire, the fire can burn for a long time. Similarly, being united with Yidden helps us serve Hashem passionately, with a hislahavus that doesn't weaken with time.
It is a Chassidic custom to sit with friends, with food and drink, sing together, and discuss avodas Hashem. Chassidim consider these gatherings extremely important because they are a moment of unity, combined with yiras Shamayim. The Sadigura Rebbe zt'l would sometimes give his chassidim a coin, and the chassidim would auction off the coin. With the money they earned from the auctions, they financed their Chassidic gatherings.
Once, the coin sold for a very high price. The chassidim wondered whether they should use all the money for their gatherings or whether part of the money should be used for some other tzedakah. The Beis Aharon zt'l (father-in-law of the Sadigura Rebbe) was then in Sadigura, so the chasidim asked the Beis Aharon for his opinion.
The Beis Aharon replied, "There is nothing greater than when chassidim gather in unity to speak about yiras shamayim..." and advised them to use the money solely for that purpose.
