بُلُوغُ الْمَرَامِ
Bulugh al-Maram 325
The Book of Prayer
وَعَنْ مُعَاذِ بْنِ جَبَلٍ - رضى الله عنه - { أَنَّ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -قَالَ لَهُ : " أُوصِيكَ يَا مُعَاذُ : لَا تَدَعَنَّ دُبُرَ كُلِّ صَلَاةٍ أَنْ تَقُولُ : اَللَّهُمَّ أَعِنِّي عَلَى ذِكْرِكَ وَشُكْرِكَ وَحُسْنِ عِبَادَتِكَ } رَوَاهُ أَحْمَدُ , وَأَبُو دَاوُدَ , وَالنَّسَائِيُّ بِسَنَدٍ قَوِيٍّ . [1]
Narrated Mu'adh bin Jabal (RA) Allah's Messenger (ﷺ) told me, "O Mu'adh, I will give you some advice - Never leave the recitation of this supplication after every prayer: 'O Allah, help me to remember You, thank You, and worship You perfectly.'" [Ahmad, Abu Dawud and an-Nasa'i reported it through a strong chain of narrators].
Sunnah.com reference: Book 2, Hadith 219