Authentic Supplication Number 145.
Abu Zumayl said: I told Ibn Abbas : I find nothing in myself – meaning no doubts. So he said to me: “Would it that you do find something in yourself then say: Huwal ‘awwalu, wal ‘akhiru, wath-thahiru, wal-batinu, wahuwa bikulli shay’in ‘aleemun (164).
(164) He is The First, The Last, The Outer, and The Inner; and He is, in everything, Ever Knowledgeable. Note: The tradition in Saheeh Muslim which appears as supplication #33, and footnote 55 is also significant for an understanding of this supplication which happens also to be verse 3 in Surat ‘Al-Hadeed (#57).
Reported by Abu Dawood, and Al-Albani said it was of comely ascription. He included this hadeeth in The Authentic of Good Sayings.