He used to teach his sons these words just as a teacher instructs young boys in writing, saying that the Messenger of Allah ﷺ used to seek refuge with them after the prayer, "Allah, I seek refuge in You from cowardice, I seek refuge in You from being returned to the most decrepit stage of life, I seek refuge in You from the trials of this world, and I seek refuge in You from the punishment of the grave."
أنه كان يعلم بنيه هؤلاء الكلمات، كما يعلم المعلم الغلمان الكتابة ويقول: إن رسول الله ﷺ كان يتعوذ منهن دبر الصلاة: اللهم أعوذ بك من الجبن، وأعوذ بك أن أرد إلى أرذل العمر، وأعوذ بك من فتنة الدنيا، وأعوذ بك من عذاب القبر.
Bukhari narrated it in the Book of Jihad and Expeditions (2822) via Abu Awana, from Abdul-Malik bin Umayr, who said, "I heard Amr bin Maymun Awdi say, 'Saad was...'" and he mentioned it.Shuba (6370, 6365), Zaida (6374), and Ubayda bin Humayd all narrated it from Abdul-Malik bin Umayr, from Musab bin Saad, from his father, but they did not mention that this was after the prayer.