Aishah said, "The Messenger of Allah ﷺ never sat in a gathering, nor recited the Quran, nor performed a prayer, without sealing it with certain words."She said, "I asked, 'Messenger of Allah, I see that you never sit in a gathering, nor recite the Quran, nor perform a prayer, without sealing it with these words.'"He replied, "Yes. Whoever has spoken good will have a seal placed upon that good, and whoever has spoken evil, they will be an expiation for him. Glory be to You and praise be to You, there is no god but You, I seek Your forgiveness, and I turn to You in repentance."
ما جلس رسول الله ﷺ مجلسا قط، ولا تلا قرآنا، ولا صلى صلاة، إلا ختم ذلك بكلمات، قالت: فقلت: يا رسول الله، أراك ما تجلس مجلسا، ولا تتلو قرآنا، ولا تصلي صلاة، إلا ختمت بهؤلاء الكلمات، قال: نعم، من قال خيرا ختم له طابع على ذلك الخير، ومن قال شرا كن له كفارة: سبحانك وبحمدك لا إله إلا أنت، أستغفرك، وأتوب إليك.
Nasai narrated it in Sunan al-Kubra (10067), from Muhammad bin Sahl bin Askar, who said, Ibn Abu Maryam narrated to us, Khallad bin Sulayman Abu Sulayman informed us, he said, Khalid bin Abu Imran, from Urwa bin Zubayr, from Ayesha, who said, and she narrated it as mentioned earlier.Ibn Abu Maryam is Saeed bin Hakam bin Muhammad bin Salim Ibn Abu Maryam Jumahi. Its isnad is hasan because of Khallad bin Sulayman and his teacher Khalid bin Abu Imran, for both of them render the hadith hasan.I say, the word "sometimes" appears in this chapter because it has not been established through any other route that the Prophet ﷺ used to conclude the Quran with it. Rather, what is well known is that he used to say that after rising from a gathering, and this is a hadith well known concerning the expiation of the gathering. See its sourcing in the section on Supplications and Remembrances.