The Messenger of Allah ﷺ said, "Whoever says in the morning, 'O Allah, I have entered the morning calling You to witness, and calling to witness the bearers of Your Throne, Your angels, and all of Your creation, that You are Allah, there is no god but You alone, with no partner, and that Muhammad is Your servant and Your messenger,' will be forgiven for whatever sins he commits during that day. And if he says it in the evening, he will be forgiven for whatever he commits during that night."
حدثنا عمرو بن عثمان حدثنا بقية عن مسلم يعني ابن زياد قال سمعت
قال رسول الله ﷺ من قال حين يصبح اللهم إني أصبحت أشهدك وأشهد حملة عرشك وملائكتك وجميع خلقك أنك أنت الله لا إله إلا أنت وحدك لا شريك لك وأن محمدا عبدك ورسولك إلا غفر له ما أصاب في يومه ذلك من ذنب وإن قالها حين يمسي غفر له ما أصاب تلك الليلة
[Hasan] Reported by Tirmidhi, Supplications, chapter, [Supplication, "O Allah, we have entered morning" or "we have entered evening, bearing witness to You and bearing witness to the bearers of Your throne..."], no. 3501, from the hadith of Baqiyyah with this wording, and he explicitly stated the chain of transmission by direct hearing throughout, in Amal al-Yawm wal-Layla, no. 9. It has a corroborating narration mentioned earlier, no. 5069. Munziri and Ibn Taymiya both quoted from Tirmidhi that he said of it, hasan.