It is narrated from Sayyiduna Mughirah bin Shu'bah (may Allah be pleased with him); he said: I heard the Prophet (peace and blessings be upon him) say: "The call of my Ummah on the Sirat (the bridge) will be: 'Rabbi sallim' (O my Lord, save us)."
Hadith Referenceالمعجم الكبير للطبراني / باب الميم / 18150