Then I later located the report in the chapter on the tashahhud of Sunan Abū Dāwūd, and in the “Book of Prayer” of al-Mustadrak (1/265), on the authority of Ibn Masʿūd, with the wording:
“The Messenger of Allah ﷺ used to teach us certain words just as he would teach us the tashahhud: ‘O Allah, unite our hearts …’ ”
Al-Ḥākim commented, “Authentic according to the criteria of Muslim,” and al-Dhahabī agreed with him.
I reply: There is room for reservation, for the chain contains Sharīk ibn ʿAbd-Allāh al-Qāḍī, who is weak on account of his poor memory. Al-Ḥāfiẓ said regarding him, “Truthful but commits many errors.”
Through this same route the ḥadith was also recorded by al-Ṭabarānī in al-Muʿjam al-Kabīr (10/236/10426); therefore, al-Haythamī’s declaration that the chain is “good” is itself unsound. The narration is listed in Ḍaʿīf Abī Dāwūd, no. 172.

His statement: “11 – ʿUmair ibn Saʿd said: Ibn Masʿūd used to teach us the tashahhud …” is transmitted by Ibn Abī Shaybah and Saʿīd ibn Manṣūr.
I note that every printed edition reads ‘Saʿd’, which is incorrect; the proper name is ‘Saʿīd’, as shown in the biographical sources and in the Muṣannaf of Ibn Abī Shaybah (1/296–297). The narrator is ʿUmair ibn Saʿīd al-Nakhaʿī al-Ṣahbānī; as for ʿUmair ibn Saʿd al-Anṣārī, he is a Companion and is not meant here.
ʿUmair ibn Saʿīd is reliable, cited by both al-Bukhārī and Muslim, and the narrators below him are likewise trustworthy, so the chain is sound.
It also appears that the full wording belongs to Saʿīd ibn Manṣūr, for Ibn Abī Shaybah does not have the phrase: “He said: No prophet ever left off …” and so on. Let the condition of that chain be examined. Ibn Abī Shaybah adds:
“Our Lord, indeed we have believed; so forgive us our sins and remove from us our misdeeds and cause us to die among the righteous. Our Lord, grant us what You promised us through Your messengers, and do not disgrace us on the Day of Resurrection—surely You never break Your promise.”