Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Allah Almighty has made a pillar of light in front of the Throne. When a servant says 'La ilaha illallah' (none is worthy of worship except Allah), that pillar begins to shake. Allah Almighty says to it: 'Be still!' It says: 'How can I be still when You have not forgiven the one who said it?' Allah Almighty says: 'I have forgiven him.' At that, the pillar becomes still."
Al-Bazzar recorded it; its chain contains Abdullah bin Ibrahim bin Abi Amr, who is extremely da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأذكار / 16804