Hadith: A mount was brought to ‘Ali (may Allah be pleased with him) to ride on, and when he put his foot in the stirrup, he said: In the name of Allah

عن علي بن ربيعة، قال: شهدت علي بن أبي طالب رضي الله عنه أتي بدابة ليركبها، فلما وضع رجله في الركاب، قال: بسم الله، فلما استوى على ظهرها، قال: الحمد لله الذي سخر لنا هذا وما كنا له مُقْرِنين، وإنا إلى ربنا لمنقلبون، ثم قال: الحمد لله، ثلاث مرات، ثم قال: الله أكبر، ثلاث مرات، ثم قال: سبحانك إني ظلمت نفسي فاغفر لي إنه لا يغفر الذنوب إلا أنت، ثم ضحك، فقيل: يا أمير المؤمنين، من أي شيء ضحكت؟ قال: رأيت النبي صلى الله عليه وسلم فعل كما فعلت ثم ضحك، فقلت: يا رسول الله، من أي شيء ضحكت؟ قال: «إن ربك تعالى يَعجب من عبده إذا قال: اغفر لي ذنوبي، يعلم أنه لا يغفر الذنوب غيري»
[صحيح] - [رواه أبو داود والترمذي وأحمد]

‘Ali ibn Rabī‘ah reported: I was there when a mount was brought to ‘Ali ibn Abi Tālib (may Allah be pleased with him) to ride on. When he put his foot in the stirrup, he said: "In the name of Allah". When he settled himself on its back, he recited: "Exalted is He who has subjected this to us, and we could not have otherwise subdued it. And indeed we, to our Lord, will surely return." He then recited: "Praise be to Allah" three times, and then: "Allah is the Greatest" three times. Then he said: "Exalted are You above any imperfection. I have wronged myself, so forgive me, for none but You can forgive sins" and then he laughed. It was asked: "O Commander of the Believers, what made you laugh?" He replied: "I saw the Prophet (may Allah's peace and blessings be upon him) doing as I have done then he laughed." I asked him: ”O Messenger of Allah, what made you laugh?” He said: “Your Lord, the Exalted, is pleased when His slave seeks His forgiveness; he (the slave) knows that none can forgive sins except Allah."
[Authentic hadith] - [Narrated by At-Termedhy - Narrated by Abu Daoud - Narrated by Ahmad]

Explanation

‘Ali ibn Rabī‘ah, who was a leading Tābi‘i, reported that he was present when ‘Ali ibn Abi Tālib was brought his riding mount. When he put his foot in the stirrup, he said: “Bismillah”, meaning: I ride in the name of Allah. When he settled on its back, he said: “Exalted is He…”, praising Him for this great favor; subduing the animal and causing it to yield to let people ride it, safe from any evil it might cause them. “…who has subjected this to us”, meaning that He has made this animal subservient to us, and we would not have been able to subdue it otherwise. Then he said: “Praise be to Allah” three times and then: “Allah is the Greatest” three times. The repetition indicates the glory of Allah, and that the slave does not give Him His true right, even though he is enjoined to worship Him at all times, according to his ability. Then he said: “Exalted are You above any imperfection. I have wronged myself…”, meaning, I have wronged myself by not fulfilling Your due right upon me in expressing gratitude for this great favor, even through slight heedlessness, a passing thought, or a glance. “…so forgive me”, meaning, conceal my sins by not punishing me for them. “…for none but You can forgive sins”, meaning that this indicates admitting negligence, despite the abundant blessings of Allah upon him. “…and then he laughed. It was asked: ‘O Commander of the Believers, what made you laugh?’" In a verified version of Ash-Shamāil, the narration says: “So I asked”, instead of: “It was asked”. The reason he was asked was that nothing had (apparently) happened that would initiate laughter. It is noteworthy that in the question, addressing him as “O Commander of the Believers” preceded the question, due to the etiquette of speech. “He replied: ‘I saw the Prophet (may Allah's peace and blessings be upon him) doing as I have done…’, referring to the riding and saying the Dhikr. “Then he laughed. I asked him, ‘O Messenger of Allah, what made you laugh?’ He said: 'Your Lord, Glorious is He, is pleased.” When ‘Ali (may Allah be pleased with him) remembered this, it necessitated even more gratitude, so he laughed. He did not laugh only by way of imitation. “…hen His slave seeks His forgiveness; he (the slave) knows”, he says this because he is aware and not heedless of the fact that: “...none can forgive sins except Allah.”