Ibn Rajab رحمه الله said:
‘Seeking forgiveness is a seal for all righteous deeds.It seals the prayer, Hajj, Qayyam al-Layl the night prayer and it seals gatherings.
Therefore, if it was a Remembrance of Allaah then it is like putting a stamp of seal on it and if there were mistakes and deficiencies then it would be an expiation for it.Likewise it is befitting to complete the month of Ramadan with seeking forgiveness.
Umar bin AbdulAziz wrote to the different regions commanding them to complete Ramadan with seeking forgiveness and giving Sadaqa al-Fitr.
Indeed Sadaqa al-Fitr is purification for the fasting person from mistakes and evil deeds and seeking forgiveness patches that which is torn from the fast due to mistakes and evil deeds.
This is why some of the previous scholars said: ‘Indeed Sadaqa al-Fitr for the fasting person is like performing the two Sajdas for forgetfulness for the prayer.
’Umar bin AbdulAziz (died 101 A.H.) رحمه الله said in his book,
say as your father Adam said:
رَبَّنَا ظَلَمْنَا أَنْفُسَنَا وَإِنْ لَمْ تَغْفِرْ لَنَا وَتَرْحَمْنَا لَنَكُونَنَّ مِنَ الْخَاسِرِينَ
They said: “Our Lord! We have wronged ourselves. If You forgive us not, and bestow not upon us Your Mercy, we shall certainly be of the losers.”
Say as Nuh عليه وسلم said:
وَإِلَّا تَغْفِرْ لِي وَتَرْحَمْنِي أَكُنْ مِنَ الْخَاسِرِينَ
And unless You forgive me and have Mercy on me, I would indeed be one of the losers.”
Say as Musa عليه وسلم said:
رَبِّ إِنِّي ظَلَمْتُ نَفْسِي فَاغْفِرْ لِي
”My Lord! Verily, I have wronged myself, so forgive me.”
Say as Dhul-Noon عليه وسلم said:
سُبْحَانَكَ إِنِّي كُنْتُ مِنَ الظَّالِمِينَ
Glorified (and Exalted) are You [above all that (evil) they associate with You]. Truly, I have been of the wrong-doers.”
[From ‘Lataif al-Ma’arif’ p.383]