I heard the Messenger of Allah ﷺ say, "The best remembrance is 'There is no god but Allah,' and the best supplication is 'All praise belongs to Allah.'"
حدثنا يحيى بن حبيب بن عربي حدثنا موسى بن إبراهيم بن كثير الأنصاري قال سمعت طلحة بن خراش قال سمعت
سمعت رسول الله ﷺ يقول أفضل الذكر لا إله إلا الله وأفضل الدعاء الحمد لله
[Hasan]. Nasai narrated it in Amal al-Yawm wal-Layla, no. 831, from Yahya bin Habib, through this same chain. Ibn Majah also narrated it, no. 3800, and Ibn Hibban graded it sahih (Al-Ihsan, 843), as did Hakim, 1/498, and Dhahabi concurred with him.