Sayyiduna Ibn Abbas (may Allah be pleased with them both) narrates: When this verse was revealed: "Say: I do not ask you for any reward for this except affection for kinship" [Surah al-Shura: 23], the people asked: O Messenger of Allah, who are those relatives of yours whose love has become obligatory upon us? He (peace and blessings be upon him) said: "Ali, Fatimah and their two sons, may Allah be pleased with them."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12207