Prophet Muhammad ﷺ (may Allah send Blessings and Peace upon him) never concluded his recitation of Al-Qur’an with “SADAQALLAAHUL ‘ADZEEM”.
‘Aishah رضى الله عنها (may Allah be pleased with her) said:
“The Messenger of Allah ﷺ never sat in a gathering, or recited Qur’an, or performed a prayer except that he would conclude that act with these words, so I asked him: ‘O Messenger of Allah, I see that you do not sit in a gathering, recite Qur’an or perform prayer except that you conclude with these words?’ He replied: ‘Yes. Whoever uttered good words, then it will be a seal for them; and whoever uttered something evil, then these words will be its expiation:
سُبْحَانَكَ وَبِحَمْدِكَ، لَا إِلٰهَ إِلَّا أَنْتَ، أَسْتَغْفِرُكَ وَأَتُوْبُ إِلَيْكَ.
SUBHAANAKA WA BIHAMDIKA, LAA ILAAHA ILLAA ANTA, ASTAGHFIRUKA WA-ATUUBU ILAIKA.
Meaning:
You are free from imperfection, and all praise is to You. There is no god worthy of worship except You. I seek Your forgiveness and turn to You in repentance.’” [as-Sunan al-Kubra (10067) by Imam an-Nasa’i. Classified as ‘Sahih’ (Authentic) by Sheikh al-Albani] https://dorar.net/hadith/sharh/210594
If we truly love Prophet Muhammad ﷺ, then we should follow his Sunnah by reading and learning as many of his hadiths as we can, and putting them into practice. We must not adopt practices that have no basis in Al-Qur’an and As-Sunnah.
I conclude with a hadith of Prophet Muhammad ﷺ:
“I have left you with two things and you will never go astray as long as you hold on to them: the Book of Allah (Al-Qur’an) and Sunnah of His Prophet (As-Sunnah of Prophet Muhammad ﷺ).” [Muwatta’ Malik]