When someone complained of an ailment
Authentic hadithعن عائشة رضي الله عنها : أنَّ النَّبيَّ صلى الله عليه وسلم كَانَ إِذَا اشْتَكى الإنْسَانُ الشَّيْءَ مِنْهُ، أَوْ كَانَتْ بِهِ قَرْحَةٌ أَوْ جُرْح، قالَ النبيُّ صلى الله عليه وسلم بأُصبعِهِ هكذا - ووضع سفيان بن عيينة الراوي سبابته بالأرض ثم رفعها- وقالَ: «بِسمِ اللهِ، تُرْبَةُ أرْضِنَا، بِرِيقَةِ بَعْضِنَا، يُشْفَى بِهِ سَقِيمُنَا، بإذْنِ رَبِّنَا».
‘Āishah (may Allah be pleased with her) reported: When someone complained of an ailment or had a sore or a wound, the Prophet (may Allah's peace and blessings be upon him) did this with his index finger – the narrator, Sufyān ibn ‘Uyaynah, touched the ground with his index finger and then raised it - and said: "In the name of Allah, the dust of our earth along with the saliva of some of us will cure our ailing people, with the permission of our Lord."
Explanation
When someone was sick ,or suffering from a wound or a sore, the Prophet (may Allah's peace and blessings be upon him) would moisten (the tip of) his index finger with his saliva then touch the ground with it, so some dust would stick to it. He would then wipe it over the wound or ailing part of the body, saying: “In the name of Allah, the dust of our earth along with the saliva of some of us will cure our ailing people, with the permission of our Lord."
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).