Authentic Supplication Number 29.

He (the Prophet) said to `Ali and Fatima: “Shan’t I guide the two of you to what has more goodness in it, for you, than a servant? When you resort to your bed, make tasbeeh [to say: Subhana ‘allahi] 33 times, tahmeed [to say: ‘Al-hamdu lillahi] 33 times, and takbeer [to say: ‘Allahu ‘akbaru] 34 times”. Ali said: Thus I have not left them since I heard them from the Messenger of Allah . It was said to him: Not even the night of [the battle of] Siffeen?He said: Not even the night of Siffeen.

Reporters.
Reported by Al-Bukhari and Muslim. Al-Albani included it in The Authentic of Good Sayings as #29.