It is narrated from Sayyiduna Zayd bin Arqam (may Allah be pleased with him); he said: The Prophet (peace and blessings be upon him) passed by a house in which Sayyidah Fatimah, Sayyiduna Ali, Sayyiduna Hasan and Sayyiduna Husayn (may Allah be pleased with them) were present, and he (peace and blessings be upon him) said: "I am at war with whoever wars against them, and at peace with whoever is at peace with them."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5012