The Successor Shahr bin Hawshab (may Allah have mercy on him) relates: We were sitting with the Mother of the Believers, Sayyidah Umm Salamah (may Allah be pleased with her), asking her about the letters (modes of recitation) of the Qur'an, when a man said: "O Mother of the Believers, I feel something within myself such that if I were to utter it, my reward would be lost, and if anyone came to know of this state of mine, my neck would be struck off." She said: "I heard the Messenger of Allah (peace and blessings be upon him); he (peace and blessings be upon him) was asked about something similar to what you have asked about, and he (peace and blessings be upon him) said: 'Such a thing (a whispering) is cast only into the heart of a believer.'"
Imam al-Tabarani (may Allah have mercy on him) said: "Only Sayf narrated this hadith from Aban."
Hadith Referenceالمعجم الأوسط / باب الحاء / 3430
Hadith Takhrij«أخرجه الطبراني فى «الصغير» برقم: 356»