One day Prophet Muhammad (PBUH) said to his companions:
Would that I can see my brothers coming toward me to the pool and welcome them with bowls filled with sweet juices. Before entering the paradise, I wish I can offer them from my Pool of Kawsar.

Upon these words, the companions said:
O Messenger of God! Are we not your brothers?

He replied:
You are my companions. And my brothers are the believers who believe in me without seeing me. Surely, I ask of my Lord to illuminate my eyes with you and the believers who believe without seeing me. (1)

In another similar hadith, it is stated:
Surely, I wished to reunite with my brothers. Whereupon his addressees said:
Are we not your brothers?

He answered:
You are my companions and brothers. After me, there will also come a community who believes in me though they did not see me.

Some time later, he said:
O Abu Bakr, wouldnt you like to love a community who loves you because they have heard that you love me? Love these whom God loves. (2)

In these Hadiths, the Prophet says that at the time of the corruption of the community, there will be his brothers. He yearns for his brothers. He dearly wants to see his brothers with strong belief who will come toward the pool while he will be beside the Pool of Kawsar, and wants to offer them water from this pool.

Therefore, the people who deserve the attention and praise of the Prophet must surely be self-sacrificing, loyal, patient, ready to make every effort for the cause of Islam and must be so courageous as not to be intimidated by any kinds of despise or condemnation and as to stick to the way of the Prophet and of his companions. These are the brothers of the prophet. He calls these my brothers and his companions my friends.

It is of great importance that these fortunate people whom Prophet Mohammad (PBUH) called my brothers believe in him and in what he brought with a strong faith.

Besides, an important quality of these people is that they love Hazrath Abu Bakr because he loved the Prophet or that they love the companions, like Hazrath Abu Bakr, who love the Messenger of God.

As for the companions, they also paid great attention to these patriots of faith and followers of the light of guidance. As it is stated that they will come beside the Pool of Kawsar before they enter the paradise and as the Prophet shows such a great longing for them, they will retain their faiths, will enter the grave as believer, and will be among the residents of the paradise. (3) That is to say, Prophet Muhammad (PBUH) informs us that they will enter the grave with belief.

Sources:
1. Ramûzul-Ehadis p. 361, 4460 hadith (Abu Nuaym, Ibn-i Omarden) See also Hak Dini IV, 2731 (In relation to the 62nd verse of Yunus Surah, while it is explained that there will be no fear for the people with closeness to God, the explanation of another similar hadith is mentioned.): Hayatus-Sahabe. II, 567-568 (With two long hadiths, the truth here is mentioned.)
2. Age. p. 461. 5719 hadith. (From Ibn-i Asakir Bera b. Azib).
3. For similar hadiths, see el-Metalibul-Aliye, 4241, 8424, Muslim, Taharet, 395 Kenul-Ummal, 345, 84.