It is narrated from Sayyiduna Abu Ayyub al-Ansari (may Allah be pleased with him); he said: When Ibrahim, the son of the Messenger of Allah (peace and blessings be upon him), died, the polytheists went to one another and said: This Sabi' (one who has abandoned his religion) has been cut off (left without offspring) tonight. Thereupon Allah Almighty revealed this surah: "Indeed, We have granted you al-Kawthar" [Surah al-Kawthar: 1] — to the end of the surah.
Hadith Referenceالمعجم الكبير للطبراني / باب الخاء / 4056