The Messenger of Allah ﷺ said, "When one of you sneezes, let him say, 'Praise be to Allah in every circumstance.' Let those present say to him, 'May Allah have mercy on you,' and let him reply to them, 'May Allah guide you and set your affairs right.'"
حدثنا عبد الله حدثنا داود بن عمرو الضبي حدثنا منصور بن أبي الأسود عن ابن أبي ليلى عن الحكم أو عيسى شك منصور عن عبد الرحمن بن أبي ليلى
قال رسول الله ﷺ إذا عطس أحدكم فليقل الحمد لله على كل حال وليقل له من عنده يرحمك الله ويرد عليهم يهديكم الله ويصلح بالكم
Its isnad is hasan. Dawud bin Amr bin Zuhayr Dabbi is trustworthy and reliable, one of the teachers of Ahmad, and Abdullah bin Ahmad also narrated from him, as here. Mansur bin Abu Aswad Laythi is trustworthy. Hakam is Ibn Utayba.Mansur's uncertainty about whether Muhammad bin Abdul-Rahman narrates it from his brother Isa or from Hakam has no effect, since he was simply hesitating between two trustworthy narrators. What has more weight is that he narrates it from Isa, as established in the hadith before this one.This hadith and the one before it are among the additions of Abdullah bin Ahmad.