Hadith: O Allah, forgive all my sins, small and great, first and last

عن أبي هريرة رضي الله عنه أن رسول الله صلى الله عليه وسلم كانَ يقولُ في سجودِهِ: «اللَّهُمَّ اغْفِرْ لي ذَنْبِي كُلَّهُ: دِقَّهُ وَجِلَّهُ، وَأَوَّلَهُ وَآخِرَهُ، وَعَلاَنِيَتَهُ وَسِرَّهُ».
[صحيح] - [رواه مسلم]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah’s peace and blessings be upon him) used to say in prostration: "O Allah, forgive all my sins, small and great, first and last, and open and secret."
Sahih/Authentic. - [Muslim]

Explanation

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger (may Allah's peace and blessings be upon him) would habitually say during prostration: "O Allah, forgive all my sins, small and great, first and last, and open and secret." This is considered a form of elaboration with respect to supplication, an act of worship. The more a person repeats supplication, the more he is engaged in worship of his Lord. Moreover, repetition of the supplication in question helps one recall all one’s sins, open or hidden, and small or great. Hence, the Prophet (may Allah's peace and blessings be upon him) expanded on sins after mentioning them in general terms. A Muslim should adhere to the supplications reported from the Prophet (may Allah's peace and blessings be upon him) for they are the most comprehensive and beneficial of all.