Sayyiduna Abu Hurayrah (may Allah be pleased with him) and Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) report this saying of the Prophet (peace and blessings be upon him): "A believer to another believer is like a building, one part of which strengthens the other."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Salih bin Nabhan, who is da'if (weak).