When doubt in belive comes to your mind, you say -
هُوَ الْأوَّلُ وَالْآخِرُ وَالظَّاهِرُ وَالْبَاطِنُ وَهُوَ بِكُلِّ شَيْءٍ عَلِيمٌ
Pronunciation
Huwal-'Awwalu wal-'Aakhiru wadh-Dhaahiru wal-Baatinu, wa Huwa bikulli shay'in 'Aleem
Meaning
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)
Source: Hasan. Abu Dawud: 5110
Read the hadith
حَدَّثَنَا عَبَّاسُ بْنُ عَبْدِ الْعَظِيمِ، حَدَّثَنَا النَّضْرُ بْنُ مُحَمَّدٍ، حَدَّثَنَا عِكْرِمَةُ، - يَعْنِي ابْنَ عَمَّارٍ - قَالَ حَدَّثَنَا أَبُو زُمَيْلٍ، قَالَ سَأَلْتُ ابْنَ عَبَّاسٍ فَقُلْتُ مَا شَىْءٌ أَجِدُهُ فِي صَدْرِي قَالَ مَا هُوَ قُلْتُ وَاللَّهِ مَا أَتَكَلَّمُ بِهِ . قَالَ فَقَالَ لِي أَشَىْءٌ مِنْ شَكٍّ قَالَ وَضَحِكَ . قَالَ مَا نَجَا مِنْ ذَلِكَ أَحَدٌ - قَالَ - حَتَّى أَنْزَلَ اللَّهُ عَزَّ وَجَلَّ { فَإِنْ كُنْتَ فِي شَكٍّ مِمَّا أَنْزَلْنَا إِلَيْكَ فَاسْأَلِ الَّذِينَ يَقْرَءُونَ الْكِتَابَ مِنْ قَبْلِكَ } الآيَةَ قَالَ فَقَالَ لِي إِذَا وَجَدْتَ فِي نَفْسِكَ شَيْئًا فَقُلْ { هُوَ الأَوَّلُ وَالآخِرُ وَالظَّاهِرُ وَالْبَاطِنُ وَهُوَ بِكُلِّ شَىْءٍ عَلِيمٌ }
Narrated Abdullah ibn Abbas: AbuZumayl said: I asked Ibn Abbas, saying: What is that I find in my breast? 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: "If thou went in doubt as to what we have revealed unto thee, and ask those who have been reading the Book from before thee." He said: If you find something in your heart, say: He is the first and the Last, the Evident and the Immanent, and He has full knowledge of all things. [Hasan (Albani). Abu Dawud: 5110]
Duas from IslamicAPI's Masnun Dua dataset (based on the Quran, hadith, Hisnul Muslim and Rahe Belayat). See the dataset