Authentic Supplication Number 11.

The Messenger of Allah ┬ásaid: “Is one of you unable to gain 1,000 merits in a day?” One of those sitting with him then asked: “How can one of us gain 1,000 merits?” He said: “By making 100 tasbeehas (12), 1,000 merits would be written for him, or 1,000 demerits would be remitted from him.”

(12) Each utterance of Subhana ‘allah is a tasbeeha.

Reported by Muslim. Al-Albani included this hadeeth in The Authentic of Good Sayings as #11.