Authentic Supplication Number 195.

`Ali bin Rabee`ata said: I witnessed Ali bin Abi Taib  when he was given a beast of burden to mount, just when he put his foot in the mount he said: In the name of Allah [ Bismillahi ], then when he had ascended upon its back, he said: All praise be to Allah [ Al-hamdu lil-lah ], then he said: Subhanal-lathee sakh-khara lana hatha wa ma kunna lahu muqrineena. Wa'inna 'la rabbina lamunqaliboona (209).

Then he said Al-hamdu lil-lahi 3 times , then he said: 'Allahu 'akbaru, 3 times , then he said: Subhanaka 'allahumma 'inne thalamtu nafsee, faghfirlee, fa'innahu la yaghfiruth-thunooba 'illa 'anta (210), then he laughed, so he was asked: O' Commander of the faithful! why did you laugh? He said: I saw the Prophet  do as I have done, then he laughed, so I asked him: O' Messenger of Allah! Why do you laugh? He said "Your Lord Glorified and Exalted is He, admires when one of His bondservants says: Lord forgive me my sins, he knows that no one else forgives sins besides Me [Allah]".

Footnotes.
(209) Surat 'Az-Zukhruf (#43) Glory be to Him who has availed this to our use, for we were not bound to it. And to our Lord surely we will be returning (V 13, 14).

(210) Glorified are You my ilah I have transgressed upon myself, so forgive me, for no one forgives sins other than You.

Reporters.
Reported by Abu Dawood, An-Nisa'ee, and At-Tirmithi, and he said it was comely-sound. Al-Albani said it was as he said, that Ibn Hibban (#2380, 2381) also said it was sound, and so did Al-Hakim, An-Nawawi, and others. He said that Al-Hakim's ascription was through another channel by way of `Ali bin Rabee`ata. As for the channel of Abu Dawood, At-Tirmithi, and An-Nisa'ee it is faulted due to 'Abi 'Ishaq 'As-Subai`ee's, who is in the chain of narration, skiping of two people between himself and 'Ibn Rabee`ata as has been certified by Ibn Hajar (see Sharh Ibn `Illan). Al-Albani included this hadeeth The Authentic of Good Sayings as #138.