Suwayd bin Ghafalah (may Allah be pleased with him) states: The last words of the adhan of Sayyiduna Bilal (may Allah be pleased with him) were: "La ilaha illallah wallahu akbar (There is no god but Allah, and Allah is the Greatest)."
(Allamah al-Haythami says:) I say: Imam Nasa'i (may Allah have mercy on him) recorded a narration via Suwayd bin Ghafalah from al-Aswad bin Yazid, who says: The last words of the adhan of Sayyiduna Bilal (may Allah be pleased with him) were: "Allahu akbar, Allahu akbar, la ilaha illallah (Allah is the Greatest, Allah is the Greatest, there is no god but Allah)." Imam al-Tabarani recorded this narration in al-Mu'jam al-Kabir, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 1864