Reference : Hisn al-Muslim 135

يقرأ قوله تعالى ((هُوَ الأوَّلُ، وَالآخِـرُ،
وَالظّـاهِـرُ، وَالْبـاطِـنُ،
 وَهُوَ بِكُلِّ شَيءٍ عَلـيم))
Huwa ‘l-Awwalu wa ‘l-Ākhir,
waẓ-Ẓāhiru wal-Bāṭin, 
wa huwa bikulli shay'in `Alīm.

(Recite the Ayat) He is the First and the Last, 
the Most High and the Most Near. 
And He is the Knower of all things (in Arabic).

Reference:
Al-Hadid 57:3, Abu Dawud 4/329. Al-Albani graded it good in SahihAbu Dawud, 
3/962.

Chapter: For one afflicted with doubt in his faith