When doubt in belive comes to your mind, you say -

هُوَ الْأوَّلُ وَالْآخِرُ وَالظَّاهِرُ وَالْبَاطِنُ وَهُوَ بِكُلِّ شَيْءٍ عَلِيمٌ

Huwal-'Awwalu wal-'Aakhiru wadh-Dhaahiru wal-Baatinu, wa Huwa bikulli shay'in 'Aleem

He is the First and the Last, the Most High and the Most Near. And He is the Knower of all things. (Sura Al-Hadid 57:3) Abu Zumayl (RA) said: I asked Ibn Abbas (RA), I have a difficult question in my mind. He asked: What is it? I replied: I swear by Allah, I cannot speak about it. He asked me: Is it something doubtful? and he laughed. He then said: No one could escape that, until Allah, the exalted, revealed: (94) So if you are in doubt, [O Muhammad], about that which We have revealed to you, then ask those who have been reading the Scripture before you. The truth has certainly come to you from your Lord, so never be among the doubters. (95) And never be of those who deny the signs of Allah and [thus] be among the losers. (Sura Yunus 10:94-95)

Reference: Hasan. Abu Dawud: 5110