The Messenger of Allah ﷺ said, "Whoever says in the morning, 'O Allah, we have entered the morning calling You to witness, and calling upon the bearers of Your Throne, Your angels, and all Your creation to bear witness that You are Allah, there is no god but You alone, with no partner, and that Muhammad is Your servant and messenger,' Allah will forgive him whatever sins he commits during that day. And if he says it when he enters the evening, Allah will forgive him whatever sins he commits during that night."
حدثنا عبد الله بن عبد الرحمن أخبرنا حيوة بن شريح وهو ابن يزيد الحمصي عن بقية بن الوليد عن مسلم بن زياد قال سمعت
إن رسول الله ﷺ قال من قال حين يصبح اللهم أصبحنا نشهدك ونشهد حملة عرشك وملائكتك وجميع خلقك بأنك الله لا إله إلا أنت الله وحدك لا شريك لك وأن محمدا عبدك ورسولك إلا غفر الله له ما أصاب في يومه ذلك وإن قالها حين يمسي غفر الله له ما أصاب في تلك الليلة من ذنب
Sourcing, hasan. Nasai narrated it in Amal al-Yawm wal-Layla, no. 9, 10, and Abu Dawud, no. 5078, from the hadith of Baqiyyah with the same wording, explicitly stating the continuous chain of hearing. Ibn Hajar graded it hasan. The hadith has a corroborating witness in Dawud, no. 5069.