Abu Yahya Hakim bin Sa'd narrates that during the Fajr prayer one of the extremists called out to Sayyiduna Ali (may Allah be pleased with him) while he was in prayer. He said: "And indeed it has been revealed to you (O Prophet) and to those before you: if you commit shirk, your deeds will surely be ruined and lost, and you will surely be among the losers." [Surah al-Zumar: 65] So Sayyiduna Ali (may Allah be pleased with him) answered him while still in prayer: "So be patient; indeed the promise of Allah is true. And let not those who have no certainty make you unsteady." [Surah al-Rum: 60]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3327