Sayyiduna Abdullah bin Salam (may Allah be pleased with him) said: I was sitting with some of the Companions of the Prophet (peace and blessings be upon him), and they said: "Who among you will go to the Prophet (peace and blessings be upon him) and ask him which deed is most beloved to Allah?" Thereupon this verse was revealed: "O you who believe, why do you say what you do not do?" [Surah al-Saff: 2]. The narrator said: Then each narrator, having heard this (surah) from the narrator before him, recited it to us in the same way from beginning to end, and each one wept while reciting it.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14003