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
(a) Kharijah bin al-Salt narrates: We entered the mosque with Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) while the imam was bowing. Sayyiduna Abdullah (may Allah be pleased with him) bowed (behind the row), and we bowed with him. He walked while bowing until he joined the row, and we remained in bowing. A man passed by and greeted him, and he said: "Allah and His Messenger have spoken the truth." When he had completed the prayer, he said: "It used to be said that among the signs of the Hour are that a man will greet another man only because he knows him personally, that mosques will be taken as thoroughfares, that a man and his wife will trade together, and that horses and women will become expensive, then become cheap, and then will not become expensive again until the Day of Resurrection."
(b) A similar report is narrated from Tariq bin Shihab via Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), and he attributed its last part to the Prophet (peace and blessings be upon him). It contains some additions and omissions.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3328