Sayyiduna Abu Hurayrah (may Allah be pleased with him) said to Ka'b al-Ahbar that the Messenger of Allah (peace and blessings be upon him) said: "Every prophet has one supplication that is answered, and every prophet hastened his supplication. But I have kept my supplication for my Ummah for the Day of Resurrection; on the Day of Resurrection it will reach whoever died without associating partners (with Allah)." Ka'b said to Sayyiduna Abu Hurayrah (may Allah be pleased with him): Did you hear this from the Prophet (peace and blessings be upon him)? Abu Hurayrah (may Allah be pleased with him) said: Yes.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20773