It is narrated from Sayyiduna Abdullah bin Salam (may Allah be pleased with him): We were sitting with the Prophet (peace and blessings be upon him), and we said: "If only we knew which deed is most beloved to Allah Almighty." Then the narrator mentioned the whole account like the hadith above. This hadith is sahih (authentic) on the condition of the two Shaykhs (Bukhari and Muslim), but they did not record it. My strongest opinion is that what led them to leave it is the narration of al-Hiql bin Ziyad, which differs from the narration of al-Walid bin Muslim and others.