A man sneezed in the presence of the Messenger of Allah ﷺ and asked, "What should I say, Messenger of Allah?"He replied, "Say, 'Praise be to Allah.'"The people asked, "What should we say to him, Messenger of Allah?"He replied, "Say to him, 'May Allah have mercy on you.'"The man asked, "What should I say to them, Messenger of Allah?"He replied, "Say to them, 'May Allah guide you and set your affairs right.'"
حدثنا خلف بن الوليد قال حدثنا أبو معشر عن عبد الله بن نجي عن عمرة بنت عبد الرحمن
عطس رجل عند رسول الله ﷺ قال ما أقول يا رسول الله قال قل الحمد لله قال القوم ما نقول له يا رسول الله قال قولوا له يرحمك الله قال ما أقول لهم يا رسول الله قال قل لهم يهديكم الله ويصلح بالكم
Its chain is hasan, on account of Abu Mashar, Najeeh bin Abdul-Rahman Sindi Madani, and Abdullah bin Naji, since there is some discussion regarding both of them. The hadith of Abu Mashar is only graded hasan because he is corroborated, for the hadith has been narrated through very many routes. See 17599, 23743, 23478, and 23447.