Musnad Abu Ya'la al-Mawsili 117:64

Abdullah ibn Salamah narrated to us, Imran ibn Khalid al-Khuza'i narrated to us, from Thabit, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to establish brotherhood between two of his companions. The night would become long for one of them until he met his brother, and when he met him, he would meet him with affection and gentleness, saying, "How were you after me?" As for the general public, three days would not pass for one of them without him knowing the news of his brother.