Hadith 17:693
Abu al-Ahwas narrated to us, from Husayn, from Ibrahim, who said: If one sneezes while he is alone, he should say: "Praise be to Allah, Lord of the worlds." Then he should say: "May Allah have mercy on us and you." For indeed, whoever hears him from the creation of Allah will bless him (say *yarhamukallah*).
حدثنا أبو الأحوص عن حصين عن إبراهيم قال إذا عطس وهو وحده فليقل الحمد لله رب العالمين ثم يقول يرحمنا الله وإياكم فإنه يشمته من سمعه من خلق الله