اللَّهُمَّ أَصْلِحْ أُمَّةَ مُحَمَّدٍ، اللَّهُمَّ فَرِّجْ عَنْ أُمَّةِ مُحَمَّدٍ، اللَّهُمَّ ارْحَمْ أُمَّةَ مُحَمَّدٍ
Transliteration
Allahumma aslih ummata Muhammadin, Allahumma farrij 'an ummati Muhammadin, Allahummarham ummata Muhammad
English meaning
O Allah, set right the Ummah of Muhammad. O Allah, relieve the Ummah of Muhammad. O Allah, have mercy on the Ummah of Muhammad.
When is it recited?
Can be recited anytime for fellow Muslims.
Source
Muslim, Iman 346, 202