'Ali bin Rabi'ah reported:
In my presence, a beast was brought to 'Ali bin Abu Talib (May Allah be pleased with him) for riding. When he put his foot in the stirrup, he said: "Bismillah (With the Name of Allah)." When he had settled himself on its back he recited: "Al-hamdu lillahil-ladhi sakh-khara lana hadha, wa ma kunna lahu muqrinin, wa inna ila Rabbina lamunqalibun. (All praise belongs to Allah Who has made this subservient to us, for we had not the strength to overpower it; and to our Rubb shall we return)." He then recited thrice: "Alhamdu lillah (Praise be to Allah)," and then three times: "Allahu Abkar (Allah is Greatest)." Then he said: "Subhanaka inni zalamtu nafsi faghfir li, innahu la yaghfirudh-dhunuba illa Anta (You are far removed from imperfection I have wronged myself, so forgive me, because none but You can forgive sins)." Then he smiled. It was asked: "Why have you smiled, O Amir Al-Mu'minin (Leader of the Believers)?" He replied: "I saw the Messenger of Allah ﷺ doing as I have done. I (i.e., Ali) asked him (the Messenger of Allah ﷺ) the reason for smiling. He ﷺ said, 'Your Rubb, Glorious is He, is pleased when His slave seeks His forgiveness. He (the slave) has firm faith that none except Allah Alone can forgive sins)'."

[Abu Dawud and At-Tirmidhi].

وعن علي بن ربيعة قال: شهدت علي بن أبي طالب رضي الله عنه أتي بدابة ليركبها، فلما وضع رجله في الركاب قال: بسم الله، فلما استوي علي ظهرها قال: الحمد لله الذي سخر لنا هذا، وما كنا له مقرنين، وإنا إلي ربنا لمنقلبون، ثم قال: الحمد الله، ثلاث مرات، ثم قال: الله اكبر ثلاث مرات، ثم قال: سبحانك إني ظلمت نفسي فاغفر لي فإنه لا يغفر الذنوب إلا أنت، ثم ضحك، فقيل: يا أمير المؤمنين من أي شئ ضحكت؟ قال: رأيت النبي ﷺ فعل كما فعلت، ثم ضحك، فقلت: يا رسول الله من أي شئ ضحكت؟ قال: “إن ربك سبحانه يعجب من عبده إذا قال: اغفر لي ذنوبي، يعلم أنه لا يغفر الذنوب غيره" ((رواه أبو داود والترمذي وقال: حديث حسن)) وفي بعض النسخ: حديث صحيح. وهذا لفظ أبي داود